SheepShaver is way faster than a 100MHz MPC 604 with fairly recent hardware and yes, speed does improve with higher MHz host cpus and more cores.
The number of cores don´t matter that much though.
The downside of the fact, that SS was optimised for speed, is that some features are missing.
The 100MHz MPC604e rating may date from the time of SS development, likely with a Pentium 4 HT running Mandriva (GNU/Linux) - thats my guess, however.
Speed rating, for example: SS MacOS 8.6 (T420, macOS 10.12.6) on the left,
vs. my good old iMac Bondi on the right hand side. upload photo
If you want to emulate a G3 or G4, you best bet will be Qemu.
Not that one would call it fast - compared to SheepShaver right now, but development is going on and astonishing progress has already been made.