So, I'm building a subclass for a player and I'm trying to increase the speed. What I'm wanting to do is make it so that it increases by 5*proficiency bonus. Is it possible to do that? Or is there a better way to add scaling speed that doesn't involve making 4 extraneous features in the subclass that only serve to increase speed?

I don’t think so. You could try adding the modifier for speed bonus by PB 5 times to the same feature, but I don’t think that’ll work.

Morifier: Bonus->Subtype: Speed->Additional Bonus Type: Proficiency.

If you do have to add 4 features you can at least hide them from the builder, sheet, and details page so nobody will know that they are there.