Well, I don't think it has to be slow because of multitasking of OS4, BACKGROUNDER (from cydia) had a similar "multitasking" feature and iphone wasn't slow with it. Actually it isn't real multitasking, it is a multi threading so fast that it can be confused with multitasking.
Anyway.... I bet this slowness isn't an bug , it is an issue made by Apple on purpose, witch will be corrected in the next minor release, but the next release won't be unlockable so easily (or at all).
I know it sounds as a conspiracy theory, but don't forget the current exploit in 313 and 4 was found by hackers long time ago, only Apple couldn't find it, (otherwise they would correct it, and we won't have the present backdoor for a functioning jailbreake and unlock.) So they "made" some bugs in the release, bugs you cannot live with, and you cannot go back either, so you will be forced to upgrade to a "corrected" 4.x.x.
just my 2 cents...