You can run Windows 3.11 decently on a 80286, a machine which can't even run XP at all. You can also use Windows 3.11 on a machine with less than 4 megs of RAM, easily. So no argument there.
But if you just bump up the RAM until XP is comfortable, maybe 64 or 128 megs, there isn't much difference between Windows 3.11 and XP in speed. (If you disable the fisher price UI in XP.) If you run more than one program at once, XP definitely starts winning. Win 3.11 is shit at multi-tasking.