[JKT]

I would expect that this would be obvious, but this isn't "normal" behaviour. It is quite possible that you are experiencing the somewhat notorious JS garbage collection bug in the current version of WebKit that OmniWeb is using. This causes OmniWeb to hog the CPU and slows not just OmniWeb down, but also the entire system. It would be beneficial to run Activity Monitor when you start to experience this slowdown and perform a sample when OmniWeb starts to hog the CPU. Send that in to OmniGroup to see if it is the JS garbage collection bug or not. Next version of OmniWeb when it finally arrives is due to fix this.

Alternatively, if it isn't that bug, it could be a corrupted history log. Try clearing your History (I'd recommend deleting the various History files in ~/Library/Application Support/OmniWeb 5 by hand when the app is closed) to see if that solves your problems.