QuickLook was the feature that induced me to upgrade from Panther to Leopard after skipping Tiger.
If you select a file or folder and depress your space bar QL shows you the text, etc. of the file (without launching any apps) or the contents of the folder, and it's so useful that I can't imagine removing it, nor do I know whether it's even possible, buried as deeply in /System/Library (the untouchable part of OS X) as it is.
I can't imagine why QL is using so much CPU time; can you tell us under what circumstances that happens?
