Well... the memory usage you're describing is pretty normal for nUI though I am working on reducing that as much as possible in nUI 6.0 -- however, even at 20meg that's nothing when it comes to memory usage even on an old laptop. Heck, I'm playing on a six year old laptop myself and my frame rate moving about the world is typically in the 60-70fps range.
Let me make this absolutely clear, nUI's memory usage has nothing to do with low frame rates. That said, there's clearly something wrong as your frame rates should not be anywhere near that low.
I would suggest you start by following the instructions in this FAQ and let's see if that helps... if not, then we'll go from there:
http://www.wowinterface.com/forums/s...ad.php?t=21716