Re: Irimi/too slow problem
Me 5'10.5" and 224lb (not proud, want to do something about it)
I've watched Izumi san do the heavy bag exercise irimi-tenkan - it's why I bought a heavy bag but now I'm 12,000 km away from the bag.
It's more a matter of timing. Depending on your sports/fitness background, and depending on composition of your body mass, you'll be able to move faster with a higher proportion of muscle than adipose tissue, but it's truly more a matter of when you move than how fast.
I think one of the quotes of O-Sensei remarks about watching the fists rather than the sword. Watch with full focus and you'll be able to decide sooner what to do.