Fun fact: ShapeShifter was virtualized, not emulated, so software ran at full speed. I used it to play Myst and other games on my Amiga. Its 68060 CPU was faster than any real Mac, so programs were screamingly fast on it.
SheepShaver was originally virtualised and not emulated. The original version ran MacOS in a virtual machine on PowerPC BeOS (BeBox and Mac that ran BeOS.) Because of this it ran at full speed and was akin to the Classic environment on early MacOS X. I used to use it to run IE5 as the BeOS browser was quite poor and PowerPC never got any ports of more modern browsers. It ran really well and could be made full screen.
I’ve got it on my Amiga 500 with Vampire 500 accelerator (so-called 68080 MMX). Pretty sure that’s nearly the fastest native 68k Mac, even if it’s an FPGA.
For a very short period of time and circumstances (Mac PPC was out but some Photoshop plugins were m68k only) an Amiga with SCSI and a 68060 was the fastest Photoshop Mac. :)