No, they're equal speed. They're both the same speed as Mudokons exactly. You can test it out.
Once, I don't rememeber how again, but I ended up with an unpossessed Slig. I just randomly watched it walk off a cliff, by it's own devices.
Idiot.

If you are hiding somewhere past the drop and keep talking to sligs as they walk, they often step too far. Works with mines, too.

That glitch is included in most walkthroughs.

Ah, that does make sense. I reckon you're right there, then, cause it also explains wht he can only possess the brew farts.