The issue here is that, in the US at least, referring to someone as "slow" is making a statement about their IQ.
There are many possible causes for the coworker's "slow"-ness here, e.g. anxiety, doubt, and high standards. Perhaps the coworker feels the need to respond only with well-researched responses and refuses to engage in speculation.
I think you misunderstood. You said "The most accurate words are actually derogatory". I proved that wrong by providing you with some accurate words that you did not know about.
The issue here is that, in the US at least, referring to someone as "slow" is making a statement about their IQ.
There are many possible causes for the coworker's "slow"-ness here, e.g. anxiety, doubt, and high standards. Perhaps the coworker feels the need to respond only with well-researched responses and refuses to engage in speculation.