I know virtually nothing about the Classic II (other than it being slower than the SE/30), so I may not be advising you properly here; but assuming your machine can run System 6.0.8, why not roll back to that instead of System 7? I mean, really. If you want to "roll back" in order to get better performance, you won't get better than System 6!

But assuming the Classic II cannot run System 6, then System 7.01 or even 7.1 would be faster than 7.5.x. I know because I've tested the speed of all these systems on my SE/30. System 6 is fastest, with 7.01 and 7.1 being second fastest (and no, there is no noticeable speed difference between 7.01 and 7.1). System 7.5.5 is slower than 7.1, and 7.6.x and 8.1 are the slowest of all these OS's.

So if you can run it, I would go with System 6.