No it doesn't have anything to do with the midi, I tried it on my laptop keyboard and it does the same thing. I use FL Studio. The best way I can describe the speed setting is how it's the main component of the wobble bass in dub step. You know how it goes from WUUUB WUUUB WUUUB to WUBWUBWUBWUB, ie a slow wave sound to a really fast one? Sorry my explanations not very technical.
Messing around yesterday I found that for certain presets this isnt a problem. For example it works for Ravers Slice 2 but not Integrated Fifth, so now I'm really curious as to what the problem is.