Judging by this person's bio, I am sure he is not actually slow, at least not as defined by IQ. You don't land those jobs and credentials by being slow. Getting a quant job for example requires being able to think fast on one's feet to answer interview questions.

I think it's more like his working memory speed is not up to his satisfaction or subjectively he feels slow, but relative to everyone else or general population, he is not actually slow, much in the same way a runway model may feel fat due to body dysmorphia, but is not actually fat.

My first real struggle with slow processing time was when I started to play competitive volleyball in high school.

the vast majority of people who try a sport will suck at it, and many are still bad even with practice, hence why so few become pro. it has nothing to do with mental slowness.

It's like when Einstein felt he was bad at physics or math. No, he was brilliant at it, but he thought he was not good enough to solve the problem he wanted to solve, which had also vexed everyone else too.