How Is Processing Speed Related to Intelligence?

Intelligence has an awkward relationship with speed. People who
perform well on intelligence tests also tend, on average, to respond
efficiently on many speeded cognitive tasks. The association is reliable
enough that speed has occupied theories of intelligence for decades. Yet the obvious interpretation—that intelligent people simply have
faster brains—turns out to be much harder to defend. The first problem is that there is no single thing called mental
speed. Reaction time includes perception, decision and motor response.
Inspection time asks how briefly information can be presented and still
be discriminated. Psychometric processing-speed tasks may require
repeated visual scanning and accurate decisions. These measures
correlate with intelligence to different degrees, and none is a direct
stopwatch on thought itself. Still, the association demands an explanation.

Complex thought has to
happen in time

One influential possibility begins with a simple constraint:
cognitive operations do not occur instantaneously. A reasoning problem may require several intermediate results. If
earlier operations take longer, less time remains for later ones.
Information produced at the beginning may also become less accessible
before it is needed again. Salthouse used these ideas to explain part of the relationship
between processing speed and age-related cognitive differences. His
limited-time and simultaneity mechanisms make a broader point as well:
speed can matter because complex cognition is a sequence whose parts
have to remain coordinated. This does not imply that the quickest responder is the best thinker.
It says that temporal efficiency can change the conditions under which
thinking occurs.

The causal arrow is the
difficult part

A broad review by Sheppard and Vernon found reliable relationships
between intelligence and several measures of information-processing
speed. But correlation leaves several possibilities open. Faster elementary processing might support more effective complex
cognition. A shared neural or genetic influence might affect both.
Executive attention could contribute to performance on both speeded and
reasoning tasks. Cognitive development could improve several processes
together. Recent theoretical work comparing processing-speed and
executive-attention accounts has sharpened this problem. Mashburn,
Barnett and Engle argue that executive attention may explain individual
differences in fluid and general intelligence better than a simple speed
account. The disagreement is valuable because it prevents the correlation from
becoming its own explanation.

Fast is not the same as
efficient

There is another complication. A person can become faster by
accepting more errors. That is why serious speed research considers accuracy and response
caution. Evidence-accumulation models can distinguish, at least
statistically, a person’s threshold for responding from other components
of performance. Two people with the same reaction time may therefore have arrived
there differently. One may accumulate evidence efficiently; another may
simply be willing to answer on less evidence. The phrase “processing speed” hides these distinctions unless the
task is specified.

Why a strong reasoner
can still be slow

Hierarchical models of cognitive ability capture both what different
abilities share and what remains distinctive about them. Processing
speed correlates with reasoning, but not perfectly. A person can therefore solve difficult novel problems accurately
while being unremarkable on a task requiring rapid repetitive visual
decisions. Under generous time limits, the difference may barely matter.
Under strict time pressure, it may become conspicuous. That profile is not an exception to intelligence theory. It is what
imperfectly correlated abilities predict. Speed matters because cognition unfolds in time. But intelligence
cannot be reduced to the time it takes a person to press a key, inspect
a stimulus or complete a page. The more interesting question is not whether faster processing and
intelligence are related. They are. It is which forms of speed
help complex cognition, through which mechanisms, and under which
demands. That question remains open enough to keep “how fast is the brain?”
from becoming a substitute for “how does intelligence work?”