[Oblique Strategy]

Have you tried deleting your History files located in User/Library/Application Support/OmniWeb 5 ?

There should be several, and you have to quit OmniWeb and move them all to the trash and empty the trash to see the effect. Restart OmniWeb and see if things are a bit snappier.

Whenever OW starts lagging, it's often due to something wrong with the History files - at least in my experience.