On my 2009 MBP I noticed a significant performance decrease going from Xcode 3 to Xcode 4. It's not unusable, but Xcode 3 was quite snappy by comparison.

You can improve performance slightly in the code editor by turning off the code folding and live issues (which I didn't really care for anyway), but the integrated interface builder is horribly slower than the one in Xcode 3.

Of course now I've upgraded to a 2011 Macbook Air and Xcode 4 performance is now...acceptable.

Do you mean that you find XCode 4 responsive? Compared to XCode 3? This isn't stopping me from getting work done but it's very sluggish compared to XCode 3 or even IntelliJ.

Running it as a 32-bit app seems to have helped a bit.

I'm on a 2007 MBP and have no such problems.