i face the same problem in 600D, while in older cameras such as 40D 450D and 350D the af works ULTRA FAST.
600d + ex 580 II assist beam = slower focus
Terrible lag on focus while the assist beam lits.
looks like the assist beam of the flash isn't that strong with 600D rather than 350D
My tests made in the same dark room with the same lens and same flash same conditions, same settings.
and whats the solution to this problem?
its a real problem when you are inside a church or a club where the light conditions are way too low and no i refuse to buy 5D 3 or 6D for better focusing, not when older cameras such as 350D and 40D are focusing instantly with a flash on.