Lots of times Xcode (seemingly randomly) starts to run extremely slowly- it can take around fifteen seconds to move an object in IB, or compiling after changing one line of code can still take up to ten seconds. I took a look at my Activity Monitor, and this is what I found:

My question is, is this normal?
You only have 2GB’s of RAM. With each update to Apple’s software, they get more memory intensive. The same goes with Safari 5.1+. So to answer your question, Yes, this is normal for a machine with 2GB’s of RAM running Lion as well as other memory intensive applications, Chrome being another.
As for Interface Builder, I have noticed this too. XIB’s are XML files, so I believe as you move an object, Interface Builder is writing it’s location on the view as it is moved, so that is a very data heavy task as well.