The ones I tried (StartIsBack, ClassicShell and Start8) weren't resource hogs(in fact Start8 uses up <1megabyte of RAM at least on my machine),so you shouldn't see any performance loss even on a relatively old rig.
I see that StartIsBack says that it re-enables the original windows 7 menu that was removed unlike other 3rd party apps that need to run as an extra service. Is StartIsBack the only one to re-enable the original code?