Processing Speed

What is processing speed?

A simple definition of processing speed, also called speed of processing, is the pace at which you handle simple mental work: noticing what is in front of you, deciding what it means and acting on it. When the work is easy, almost anyone can get it right, so what separates people is how fast they get through it without mistakes. In plain terms, as the US nonprofit Understood.org puts it, slow processing speed "is when people need a lot of time to take in, make sense of, and respond to information" (Understood.org).

The simplest measure is reaction time. A 2015 study of 1,469 adults called simple reaction time, "the minimal time needed to respond to a stimulus", "a basic measure of processing speed" (Woods and others, 2015). IQ tests use slightly longer tasks that last a minute or two and add a little thinking to the speed.

Two kinds of mental speed

The Cattell-Horn-Carroll (CHC) model, the map of abilities behind most modern IQ tests, separates the two. They are "called processing speed (Gs) and decision/reaction time speed (Gt)" (Schmitz and Wilhelm, 2019).

You can try a free processing speed test of each kind below. Neither is a standardised test, so neither gives you an index score, but both show what these tasks feel like and how your speed compares with research figures.

Free processing speed test: 60-second symbol coding

This is modelled on digit-symbol coding, one of the oldest tasks in psychology and part of the processing speed index on today's Wechsler tests. The key pairs nine symbols with the digits 1 to 9. Each time a symbol appears, type or tap its digit. You have 60 seconds. The key changes every time you play, so memorising it from a previous try won't help.

Free reaction time test

Click or tap the panel to start. When it turns green, click as fast as you can. You get five tries, and the result is your median, the middle of the five, which is less affected by one slip than an average.

For comparison, in a carefully calibrated lab study of 1,469 adults aged 18 to 65, the average simple reaction time was 231 milliseconds, or 213 milliseconds once the delay added by the computer hardware was removed (Woods and others, 2015). Phones, touchscreens, wireless mice and busy browsers all add lag of their own, so an online result is usually a little slower than your true speed. How much reaction time runs in families is covered in is intelligence hereditary?

Processing speed vs processing ability

Psychologists separate two kinds of test. The American Psychological Association defines a speed test as one that counts "the number of problems or tasks the participant can solve or perform in a predesignated block of time", and a power test as one that measures "level of mastery ... under conditions of little or no time pressure", with items that "become progressively more difficult" (APA, speed test; APA, power test).

Processing speed is the first kind. It asks how much easy work you can do, not how hard a problem you can crack. Reasoning ability, such as solving matrix puzzles or explaining what words mean, is closer to the second. The two are related, but they can be far apart in the same person. In a sample WISC-V report published by Pearson, a child scored 133 on the Verbal Comprehension Index and 100 on the Processing Speed Index. Even 100 is perfectly average, yet it was a clear relative weakness for that child, and only 9% of children in the test's norm sample showed a gap that large (Pearson, WISC-V sample score report).

What this looks like in daily life: someone with strong reasoning and average speed often understands a topic well but needs longer to finish timed exams, copy notes or write answers out. Extra time helps them far more than extra explanation.

The Processing Speed Index (PSI)

The APA describes the Processing Speed Index as "an index from more recent versions of the Wechsler Adult Intelligence Scale and other Wechsler tests that measures the speed of nonverbal processing" (APA Dictionary). It is one of five primary index scores, alongside verbal comprehension, visual spatial, fluid reasoning and working memory, explained in our guide to the Wechsler IQ test.

Sources: Pearson WAIS-5 overview; Pearson WISC-V interpretive report.

Pearson's report describes the PSI as measuring "speed and accuracy of visual identification, decision making, and decision implementation", and lists what goes into it: "visual scanning, visual discrimination, short-term visual memory, visuomotor coordination, and concentration". That last list matters. A low score can come from slow scanning, but also from a weak pencil grip, poor eyesight or a wandering mind, which is one reason the WAIS-5 also offers a motor-reduced speed index.

What do processing speed scores mean?

Like the full scale IQ, the PSI has an average of 100 and a standard deviation of 15, and "scores ranging from 90 to 109 are typically considered average". The labels below are the ones Pearson uses for the WISC-V.

Shares follow from the normal curve with mean 100 and SD 15. Any index score can be read the same way as an IQ score: see, for example, what a score of 85 means or what 115 means.

What does a low processing speed score mean?

On its own, very little beyond "this person works through simple timed tasks more slowly than most people their age". It says nothing about how well they understand things, and it is not a diagnosis. Understood.org notes that slow processing speed by itself "typically doesn't make a child eligible for an IEP or a 504 plan", the US school support plans (Understood.org).

It is, however, a common companion to several conditions, so psychologists treat it as a clue:

• ADHD. "It is well established that processing speed is negatively impacted in children and adolescents with Attention-Deficit/Hyperactivity Disorder" (Cook and others, 2018).
• Dyslexia. A study comparing the two groups found "a general PS deficit exists in both clinical groups compared to controls, although children with RD [reading disability] demonstrate greater PS deficits than children with ADHD", and suggested slow speed may help explain why the two conditions so often occur together (Shanahan and others, 2006).
• Autism. In 56 children with autism and IQs above 70, the subtests that make up the processing speed index were among the relative weaknesses, and slower speed went with more communication difficulties (Oliveras-Rentas and others, 2012).
• Brain injury and illness. Digit-symbol coding has been called "the single most sensitive test, among WAIS subtests, to determine the presence of brain damage", though it can't say what kind (Jaeger, 2018).

Ordinary causes matter too: tiredness, stress, illness, some medicines and simply being unfamiliar with the task all slow people down. If you are worried about a child's or your own speed, a psychologist can test it properly and look at the whole profile.

Processing speed and intelligence

The idea that quick minds are clever minds is old. In 1884, at the International Health Exhibition in London, Francis Galton ran an "anthropometric laboratory" that measured visitors' reaction times to sights and sounds, among other things, and collected data on 9,337 people in about a year (Johnson and others, 1985).

Modern studies find a real but moderate link. In a representative sample of 900 Scottish adults aged 56, intelligence test scores correlated −0.31 with simple reaction time and −0.49 with four-choice reaction time (Deary, Der and Ford, 2001). The minus sign means faster people tended to score higher, and the link was stronger when each response needed a decision. Inspection time, how long you need to see which of two lines is longer, correlates about −0.5 with IQ in reviews of the research (Posthuma and others, 2001).

A correlation of 0.3 to 0.5 is meaningful, but it leaves plenty of room for slow, careful people who reason brilliantly and quick people who don't. That is why IQ tests treat speed as one part of the picture. Our free IQ test has a time limit, but it scores the four kinds of reasoning, not how fast you click.

How fast is the brain?

• 13 milliseconds. People shown a rapid stream of photos could still pick out pictures matching a description when each one was on screen for just 13 ms. MIT's report on the study put it as "the human brain can process entire images that the eye sees for as little as 13 milliseconds" (Potter and others, 2014; MIT News).
• Under 150 milliseconds. Shown a photo they had never seen for only 20 ms, people decided whether it contained an animal, and brain recordings showed the decision taking shape about 150 ms after the photo appeared (Thorpe, Fize and Marlot, 1996).
• About 10 bits per second. Yet the speed of thinking, measured as information we act on, is surprisingly slow. A 2025 paper estimated that "the information throughput of a human being is about 10 bits/s", while our senses take in around a billion bits per second (Zheng and Meister, 2025). Seeing is fast; deciding what to do, one thing at a time, is the bottleneck.

Processing speed and age

Speed is the first ability to peak. In data from 48,537 people tested online, plus the norms of standard IQ and memory tests, digit-symbol coding peaked in the late teens, well before working memory around 30 and vocabulary in later middle age (Hartshorne and Germine, 2015). Our guide to fluid vs crystallized intelligence charts these peaks side by side.

The psychologist Timothy Salthouse argued that this gradual slowing explains much of the change in thinking with age. His theory holds that "increased age in adulthood is associated with a decrease in the speed with which many processing operations can be executed and that this reduction in speed leads to impairments in cognitive functioning": steps that run too slowly either don't finish in time, or the results of earlier steps fade before later ones are done (Salthouse, 1996).

The slowing is gentle. Simple reaction time rose by only about half a millisecond a year (0.55 ms) between 18 and 65, mostly because of slower movement rather than slower seeing, and the authors concluded that "processing speed is as fast in contemporary populations as in the Victorian era", when Galton measured it (Woods and others, 2015). Because IQ and index scores compare you with people your own age, a 70-year-old with average speed for 70 still scores about 100.

How processing speed is measured

• Digit-symbol coding. Copy symbols that go with numbers, as in the task above. Versions date back "to at least 1900", and the test was part of the first Wechsler scale in 1939 (Jaeger, 2018).
• Symbol search and cancellation. Scan rows or pages for target symbols or pictures and mark them.
• Trail Making Test. Join 25 numbered dots in order (part A), then alternate numbers and letters, 1–A–2–B (part B). Developed in 1938 by John Partington and Russell Leiter (APA Dictionary).
• Reaction time tasks. Simple (one signal, one response) and choice (several signals, each with its own response), measured on a computer in milliseconds.
• Inspection time. The shortest display needed to judge something simple, such as which of two lines is longer. It needs no fast movement at all, which makes it a purer measure of perceptual speed.

"Processing skills" is used more loosely, usually to mean the whole set of ways the brain handles incoming information: visual processing (covered in our guide to spatial reasoning), auditory and language processing (see verbal reasoning), working memory and processing speed.

Can you improve your processing speed?

You can get faster at speed tasks, and some of the gains last. The best evidence comes from ACTIVE, a US trial that randomly assigned 2,832 people aged 65 to 94 to ten sessions of memory, reasoning or speed-of-processing training, or to no training. Each kind of training improved the ability it targeted, by an amount equal to the decline expected over 7 to 14 years (Ball and others, 2002). Ten years on, the speed and reasoning groups still showed gains, and the speed group reported less difficulty with everyday tasks (Rebok and others, 2014).

The trial has also been linked to dementia risk, though here the evidence is more tentative. After ten years, the speed-trained group had a 29% lower risk of dementia, a result that only just reached statistical significance (Edwards and others, 2017). A 2026 analysis of the same people's medical records over 20 years found a lower risk only for those who had also taken booster sessions (ACTIVE 20-year follow-up, 2026).

Other things that help:

• Aerobic exercise. Across 29 randomised trials, exercise training led to "modest improvements in attention and processing speed" (Smith and others, 2010).
• Sleep. A night without sleep hits simple attention hardest and slows processing speed too, while reasoning accuracy suffers least (Lim and Dinges, 2010). Try the coding task after a bad night to see it for yourself.
• Action video games. Playing them "significantly reduces reaction times without sacrificing accuracy" in lab tasks (Dye, Green and Bavelier, 2009).

What none of this shows is that getting faster raises your IQ in general. Training mostly improves the skills you practise, a pattern also found for memory training, as our fluid intelligence guide explains. Our guide to increasing your IQ sets out what has and hasn't raised scores.

Frequently asked questions

Sources

• American Psychological Association, APA Dictionary of Psychology: Processing Speed Index, speed test, power test, Trail Making Test, levels-of-processing model of memory.
• Pearson. WAIS-5 overview; WISC-V interpretive report (sample); WISC-V score report (sample).
• Understood.org. Processing speed: what you need to know; Can my child get an IEP for slow processing speed?
• Schmitz, F. and Wilhelm, O. (2019). Mene mene tekel upharsin: clerical speed and elementary cognitive speed are different by virtue of test mode only. Journal of Intelligence, 7(3), 16.
• Woods, D. L. and others (2015). Factors influencing the latency of simple reaction time. Frontiers in Human Neuroscience, 9, 131.
• Deary, I. J., Der, G. and Ford, G. (2001). Reaction times and intelligence differences: a population-based cohort study. Intelligence, 29(5), 389–399.
• Posthuma, D., de Geus, E. J. C. and Boomsma, D. I. (2001). Perceptual speed and IQ are associated through common genetic factors. Behavior Genetics, 31(6), 593–602.
• Johnson, R. C. and others (1985). Galton's data a century later. American Psychologist, 40(8), 875–892.
• Thorpe, S., Fize, D. and Marlot, C. (1996). Speed of processing in the human visual system. Nature, 381, 520–522.
• Potter, M. C., Wyble, B., Hagmann, C. E. and McCourt, E. S. (2014). Detecting meaning in RSVP at 13 ms per picture. Attention, Perception, & Psychophysics, 76(2), 270–279.
• Zheng, J. and Meister, M. (2025). The unbearable slowness of being: why do we live at 10 bits/s? Neuron, 113(2), 192–204.
• Hartshorne, J. K. and Germine, L. T. (2015). When does cognitive functioning peak? Psychological Science, 26(4), 433–443.
• Salthouse, T. A. (1996). The processing-speed theory of adult age differences in cognition. Psychological Review, 103(3), 403–428.
• Cook, N. E., Braaten, E. B. and Surman, C. B. H. (2018). Clinical and functional correlates of processing speed in pediatric ADHD: a systematic review and meta-analysis. Child Neuropsychology, 24(5), 598–616.
• Shanahan, M. A. and others (2006). Processing speed deficits in attention deficit/hyperactivity disorder and reading disability. Journal of Abnormal Child Psychology, 34, 584–601.
• Oliveras-Rentas, R. E. and others (2012). WISC-IV profile in high-functioning autism spectrum disorders. Journal of Autism and Developmental Disorders, 42, 655–664.
• Jaeger, J. (2018). Digit Symbol Substitution Test: the case for sensitivity over specificity in neuropsychological testing. Journal of Clinical Psychopharmacology, 38(5), 513–519.
• Craik, F. I. M. and Lockhart, R. S. (1972). Levels of processing: a framework for memory research. Journal of Verbal Learning and Verbal Behavior, 11, 671–684.
• Ball, K. and others (2002). Effects of cognitive training interventions with older adults: a randomized controlled trial. JAMA, 288(18), 2271–2281.
• Rebok, G. W. and others (2014). Ten-year effects of the Advanced Cognitive Training for Independent and Vital Elderly cognitive training trial on cognition and everyday functioning in older adults. Journal of the American Geriatrics Society, 62(1), 16–24.
• Edwards, J. D. and others (2017). Speed of processing training results in lower risk of dementia. Alzheimer's & Dementia: Translational Research & Clinical Interventions, 3, 603–611.
• Impact of cognitive training on claims-based diagnosed dementia over 20 years: evidence from the ACTIVE study (2026). Alzheimer's & Dementia: Translational Research & Clinical Interventions, 12.
• Smith, P. J. and others (2010). Aerobic exercise and neurocognitive performance: a meta-analytic review of randomized controlled trials. Psychosomatic Medicine, 72(3), 239–252.
• Lim, J. and Dinges, D. F. (2010). A meta-analysis of the impact of short-term sleep deprivation on cognitive variables. Psychological Bulletin, 136(3), 375–389.
• Dye, M. W. G., Green, C. S. and Bavelier, D. (2009). Increasing speed of processing with action video games. Current Directions in Psychological Science, 18(6), 321–326.