Processing speed sounds like it would be processing speed, such as how fast one can mentally assess or manipulate concepts or information. That is difficult to measure. The subtests on the wppsi and wisc that feed into the processing speed index can not measure the mental component alone, they necessarily also measure how fast the child is visually able to input information including how well they can keep their place on a page, their attention span, how well they listened to the instructions that were given them, how fast they can manipulate a pencil, how much they like to check their own work, how much of a perfectionist they are, etc. Those are timed subtests and some friendly kids may be too chatty to use their time effectively. Others may feel distractedly anxious knowing they are being timed. Those subtests also to me appear to favor a quick and dirty get the job done approach, versus detail oriented careful work. For example in one of the processing speed subtests on the test my son took they are to draw some simple shapes, a child who likes all their circles to look very round might end up much slower than another who is content with anything that remotely resembles an oval.
It's apparently very normal for a child who is otherwise very gifted to be more average in processing speed. They do say that when there are standard deviations between one or more other indexes and the "processing" speed, that the child may not appear to teachers or strangers to be as gifted as they seem at home. And it can be a bottleneck that frustrates the child internally too. This is absolutely true for my son who's processing speed index is 50 or more points lower than his other indexes. So that's something to watch out for.
