The original 68K emulator in PPC Mac ROMs was an interpreting emulator and even in a loop, it would reinterpret the instructions over and over. This cost the emulator a lot of performance.
Connectix wrote a caching interpreter which would keep chunks of the interpreted code and reuse them as needed, thus vastly speeding up loop structures and other repetitive chunks of code.
I think that with Connectix's Speed Doubler installed, an 80MHz PPC601 was pretty comparable to the fastest 68040.
In the PCI PowerMacs, Apple copied (or licensed?) Connectix's code and used that as the built-in 68K interpreter so a huge performance boost was seen beyond the improvements brought with faster CPUs, and more efficient memory subsystems.
If you're running a NuBus PPC or a PPC upgrade in a 68K machine, get Speed Doubler and install it. If you're using a PCI PowerMac there's little need for Speed Doubler (though some of the disk management/copy enhancements are nice).