First of all, have you looked under the File menu? There's now a "Quit Other
Applications" menu item that should do exactly what you're after. If you
don't want to do that for some reason, read on:
"Niceness" is one of those Unix concepts, and it didn't really apply (or
have exactly the expected effect) on a cooperative system like MacOS. To
give the program more time at the expense of other apps, you want to
increase the "FGRUN" and "BGRUN" values in the prefs file. Basically
increasing FGRUN will just decrease the rate at which it polls for mouse
clicks when it's in front. Increasing BGRUN will mean other programs get
less time when deschall's in the background, which can make the frontmost
program unresponsive. If you want to go all out, the program should really
be in front anyway, so only FGRUN will come into play.
Andrew