OK, I got curious enough I wrote a small console game to test reaction times.
It turns out I'm not nearly as good as I thought. I can nail a delay of 100,200,...,900ms exactly only about 40% of the time, and within +-100ms about 80% of the time.
If anyone else feels like trying, you can do "go get github.com/erinok/howlonghowlong". Hit space to see a flash, and then type 1-9 for how long you think the flash was.
It's surprisingly fun. (At least if you're incredibly weird like me.)
I find I'll have periods of concentration where I can just nail it, and then it'll leave me, and I'll be sloppy.