Processing Speed: What It Is, the Processing Speed Index and How It Relates to IQ

By the IQ Revealed Team · Updated October 27, 2026

Processing speed is how quickly you can take in simple information, make a decision about it and respond: spotting whether a symbol appears in a row, copying a code, reacting to a light. It is the pace of easy mental work, not the ability to solve hard problems. Intelligence tests measure it with short timed tasks and report it as the Processing Speed Index. It is moderately related to IQ, which means a slow score is not a sign of low intelligence, but the two are not unrelated either.

Processing speed at a glance

What it is: the speed and accuracy of simple perceptual and decision tasks.
How it is measured: timed tasks such as Coding and Symbol Search on the Wechsler scales, and reaction-time tasks in research.
Its score: the Processing Speed Index (PSI), mean 100, standard deviation 15, like the other index scores.
Link to IQ: moderate. Faster people tend to score higher, but many strong reasoners are not especially fast.
Across life: rises through childhood, peaks in early adulthood, then slows gradually.
Not a diagnosis: “slow processing speed” describes a pattern; it does not explain it.

What processing speed means

Psychologists use the term for the rate at which a person can carry out mental operations that are easy in themselves. Nobody finds it hard to decide whether two shapes match. What differs between people is how many such decisions they can make, accurately, in a minute. That is why processing speed tasks are deliberately simple: if the items were difficult, the score would reflect reasoning rather than speed.

In the Cattell-Horn-Carroll model of abilities that most modern intelligence tests are built around, processing speed is one of the broad abilities, alongside reasoning, comprehension-knowledge, working memory and visual processing. Pearson’s description of the WISC-V names verbal comprehension, visual spatial, fluid reasoning, working memory and processing speed as its five primary index scores. Our article on types of intelligence explains how these broad abilities fit together, and working memory, the neighbouring index, is often confused with speed.

How the Processing Speed Index is measured

On the Wechsler scales, the Processing Speed Index comes from short timed tasks:

Coding: a key pairs simple symbols with numbers or shapes, and the person copies the matching symbol under each item as fast as they can.
Symbol Search: the person looks at one or two target symbols and marks whether either appears in a nearby group.
Cancellation: a supplemental task in which the person marks every target item in a busy array.

The PSI is reported on the same scale as IQ: a mean of 100 and a standard deviation of 15, so about two-thirds of people score between 85 and 115. It is one of four index scores on the WAIS-IV and one of five on the WISC-V and the WAIS-5. The IQ scale explains what any score on this metric means as a percentile.

Because these tasks are timed, they also depend on things that are not thinking speed at all: fine-motor control, eyesight, attention, anxiety and how carefully a person checks their work. A slow but perfectly accurate performance and a fast careless one can earn similar scores. Pearson’s own WISC-V materials raise the related question of children who reach correct answers just after a time limit on the reasoning tasks, which is one reason examiners read speed scores alongside the rest of the profile rather than in isolation.

Processing speed and IQ

One answer at the top of search results says processing speed has no relation to intelligence. The research says otherwise, though the relation is moderate rather than strong.

Geoff Der and Ian Deary studied 2,196 adults from the West of Scotland Twenty-07 study, aged 30, 50 and 69, who took a general intelligence test and two reaction-time tasks. Simple reaction time correlated -0.27, -0.30 and -0.32 with the intelligence test in the three age groups, and four-choice reaction time -0.44, -0.47 and -0.53. The minus sign means faster responders scored higher. Two further details matter:

The link grew with age. It was strongest in the 69-year-olds.
It was not a straight line. The association was stronger at the lower end of the intelligence scores, so among high scorers speed told you less.

A correlation around -0.5 leaves most of the variation in one measure unexplained by the other. In practice, that means plenty of people reason well but work slowly, and the reverse. On the Wechsler scales the Full Scale IQ includes a processing speed subtest, which is why examiners also report a General Ability Index that leaves out speed and working memory when those scores sit far below reasoning. Our article on twice-exceptional people covers that profile, and the g factor explains why most abilities, speed included, correlate with one another.

How processing speed changes with age

Robert Kail pooled 72 studies and 1,826 pairs of reaction times comparing children or adolescents with adults. Younger groups were slower by a consistent factor across very different tasks, and that factor shrank with age along an exponential curve: speed rises quickly in childhood and more slowly in adolescence. The consistency across tasks suggested a general, not task-specific, change.

In adulthood the direction reverses. Timothy Salthouse’s processing-speed theory proposes that slowing with age is a main reason other abilities decline: when processing is slow, some steps cannot be finished in time, and early results may be lost before later steps are done. He reported that statistically controlling for speed removes a large part of the age differences on other cognitive measures. This is why speed falls earlier and faster than vocabulary, a pattern our article on fluid vs crystallized intelligence describes.

Slow processing speed

“Slow processing speed” is a description, not a diagnosis. It is not a condition listed in the DSM-5-TR. It describes someone who completes simple timed work more slowly than most people of their age, on tests or in daily life: taking longer over homework or routine tasks, needing more time to answer, or falling behind in fast conversation.

It often appears alongside other things. In children with ADHD, Susan Mayes and Susan Calhoun found that working memory and processing speed were the lowest index scores: every child in their WISC-IV sample of 118 scored lowest on one of the two. But slow performance on timed tasks has many possible sources, from tiredness, illness or anxiety on the day to normal ageing, so the pattern alone cannot tell anyone which one applies. Speed problems that are new, worsening or affecting school or work deserve an assessment by a qualified professional, such as a psychologist or doctor.

Can processing speed be improved?

People get faster at tasks they practise. The largest test of this is the ACTIVE trial, which randomly assigned 2,832 adults aged 65 to 94 to memory, reasoning or speed-of-processing training, or to no training. After ten sessions, 87% of the speed-trained group showed reliable improvement on the trained ability, more than in the memory and reasoning groups, and booster sessions added to the gain, which lasted two years. But the authors found no training effect on everyday functioning at two years.

That is the usual pattern for brain training: gains on the trained task, limited transfer beyond it. Our guide on how to increase IQ covers the transfer problem, and dual n-back is another well-studied example. What reliably helps day to day is less about raising speed than working with it: enough sleep, fewer distractions, breaking long instructions into steps and, where a professional recommends it, extra time on tests.

Trust and scope notes

This page summarises published research for general information. The figures quoted are those reported by the studies listed below. It explains what processing speed means in general and does not interpret any individual’s scores; only a qualified professional who has assessed a person can do that. The IQ Revealed test is an online reasoning test of 40 original items for adults aged 18 and over. It does not measure processing speed, it cannot diagnose, screen for or rule out any condition, and it is not a clinical assessment. Our guide to how IQ tests work explains what an online test can and cannot tell you.

IQ Revealed is not affiliated with, endorsed by or connected to Pearson, the publishers of the Wechsler scales (WAIS, WISC), or the researchers, universities, studies and journals named on this page. Test names are used only to identify the instruments being described.

Frequently asked questions

What is processing speed?＋

Processing speed is how quickly a person can take in simple information, make a decision about it and respond, for example scanning a row of symbols for a match or copying symbols that are paired with numbers. It is about the pace of easy mental work, not the ability to solve hard problems. Intelligence tests such as the Wechsler scales measure it with short timed tasks and report it as the Processing Speed Index.

What does the Processing Speed Index measure?＋

The Processing Speed Index (PSI) is one of the index scores on the Wechsler intelligence scales. It is built from timed paper-and-pencil or tablet tasks, usually Coding and Symbol Search, that are easy for almost everyone, so the score reflects how many items a person completes accurately in the time allowed. Like the other index scores it has a mean of 100 and a standard deviation of 15.

Is processing speed related to IQ?＋

Yes, moderately. Faster performance on simple speed tasks goes with higher scores on intelligence tests: in a Scottish study of 2,196 adults, four-choice reaction time correlated between -0.44 and -0.53 with a general intelligence test, the minus sign meaning faster people scored higher. But the link is far from perfect, so a slow processing speed does not mean low intelligence, and many people with strong reasoning have a relatively low processing speed score.

What does slow processing speed mean?＋

It means a person completes simple, timed mental tasks more slowly than most people of the same age. It is a description of a pattern in test scores or daily life, not a diagnosis in itself. In children with ADHD, processing speed and working memory are often the lowest index scores, but slow timed performance has many possible sources, including tiredness, anxiety and normal ageing. Only a qualified professional who has assessed the person can say what explains it.

Can processing speed be improved?＋

Practice makes people faster at the specific tasks they practise. In the ACTIVE trial of 2,832 older adults, 87% of those given speed-of-processing training showed reliable improvement on the trained ability, and the gain lasted two years. But no effect on everyday functioning was found at two years, and there is no good evidence that speed training raises general intelligence.

At what age is processing speed fastest?＋

Processing speed rises quickly through childhood, more slowly through adolescence, and is generally fastest in early adulthood. It then slows gradually across adult life. Psychologist Timothy Salthouse has argued that this slowing explains a large share of the age-related decline in other kinds of thinking.

Does the IQ Revealed test measure processing speed?＋

No. Our test is an online reasoning test of 40 original items for adults aged 18 and over. It does not produce a processing speed score and it is not a clinical assessment. A Processing Speed Index comes only from an individually administered test such as the Wechsler scales, given by a qualified professional.