[troyb]

The full text of web pages is stored in an ox database which unfortunately isn't the greatest performer at larger sizes. It's this database that usually causes the history slow downs and there isn't a lot that can be done to prevent this from happening.

You can nuke your history index database. Doing this wont kill your history list just the ability to do full text searches of the visited pages (which it sounds like you may be using).

1.) In finder go to: "~/Library/Application Support/OmniWeb 5/" (where ~ is your home folder)
2.) Find and remove the file called: "HistoryIndex.ox"

Restart OmniWeb.