As for the SuperFX.
The SNES has an infamously slow single CPU at 3.6 Mhz for a total of 1.5 MIPS (the console was basically finished design-wise two years before they released it...same as N64), the Genesis and Neo-Geo both have two as they have a Z80 included at roughly 4Mhz to handle basic traffic management, they have the same main CPU but the Genesis is clocked at 7.7 Mhz (for 1.4 MIPS by itself) and NG's at 12 Mhz (2.1 MIPS). This is why early and many SNES games slowdown massively when they hit what should be no problem sprite limits.
The "solution" for this were three separate CPU's put inside the cartridges themselves.
To boost anything that used Mode 7 along with a few other games they included the DSP which ran at 8 Mhz. This basically works more like a GPU add-on.
There's the two models of the SuperFX, both of which run at
21.5 Mhz but the first model is clock halved, and do 10 MIPS and 21 MIPS respectively. The SuperFX2 actually does not have any extra features other than it runs at the full clock and I think there might be some manufacturing-related savings. People have removed the clock limiter on Star Fox and Stunt Race FX cartridges which let their FX run at the full speed. IIRC, you only have to remove a couple pins or something.
Then there's the SA-1. This is basically the same base CPU that's in the SNES with three years of enhancements plus it's clocked at 10.8 Mhz and it also has improved compression methods along with a tiny bit of internal RAM cache. Super Mario RPG uses this.
In the case of both of the latter two, the original SNES CPU is either ignored or used like the Z80 in the Genesis/Neo-Geo, the in-cart CPU basically does everything otherwise. If you remember the old ZSNES vs. Snes9x days, this was the big fight over emulating games with these, especially the SA-1. IIRC, they never did, it's all hacks and SA-1 was assumed to be some kind of enhancement chip like DSP. Then Byuu came along trying to do cycle-accurate emulation and confirmed that it's actually used as the main CPU like the FX, which is part of why the Snes9x developers came back to release a new version after years of nothing last year.
Oddly enough, Sega only ever did this for Virtua Racing and like over did it which is why they never did it again, as the SVP runs at 23 Mhz for 25 MIPS but cost Sega a shit load since it wasn't a simple little chip like the SA-1 and FX are and it runs alongside the CPU. The SegaCD has the same CPU as Genesis just clocked up to 12.5 Mhz, but as DFRetro showed in their 32X video, almost none of the games use both CPUs (or in the case of 32X CD all the CPUs) but pick one or the other.
Which when you think about, that image of the Sega Tower where someone has plugged in everything including 32X, game genie, sonic and knuckles, etc. and it works, probably only like one part of the hardware is actually being used the rest is all pass through data.
