On the X100 machines Speed Doubler is much more than disk caching software. It changes the 680x0 code emulator from an interpreting emulator (translates each instruction as it goes, if it's in a loop, every instruction will be translated every time through the loop) to more of a compiler, by which the 680x0 code is converted to PPC and a chunk of it is retained in memory. If the program loops, most of the instructions' translations are still around and the translation needn't be repeated.

This can do things like double the speed of emulation of 680x0 code.

On later machines (x500, x600) Apple used the same kind of scheme; they may have licensed Connectix's code, I'm not sure.

So the place Speed Doubler really shines is on X100 machines. Sure, it still has some nifty copy and disk caching functions on later machines, but it really speed things up on the 6100, 7100, 8100 and related clones.