Oh, was messing around on W4 the other day (lost at wave 800ish, grade 103 couldn't keep up with 10^52), and although it would be feasible to say lose freeze altogether, it gets laggy besides tedious to keep up. My cpu is pretty wimpy by today's standards, but once critters hit 10^22+ hp, 1000s of them on screen lag something somewhere that either same number of monsters that take no damage or die in one hit don't.
Grabbing the mana first and then killing them off screen lags tons less (minor stutter occasionally), probably not as noticeable on newer hardware and an OS that has a better implementation of flash.