I put 40T sprockets on all my bikes... they seem to be a good compromise between speed & low end power that works well with most HT engines.
I've used 36T's on a few bikes, & yes they're faster on the flat, but you lose a lot of pull going up hills.
After riding these bikes for a few years, I've gotten over the need to go real fast every day. Now I like to cruise & enjoy the ride!
The only time I ever need to go real fast is on our motorized bicycle club rides... (If you aren't fast, you get left in the dust!!)