If you scroll down in that settings screen to the Database section, there should be a message which reads "You have [x] projects and [y] actions stored in [z] zip files". How many zip files does it list?
If you have more than 100 or so, that's probably the reason for the slowness. If you're not synchronizing, try pressing the Compact Database button to consolidate your database history into a single zip file. (The next release of OmniFocus for iPhone will start doing this automatically when appropriate.)
If you have a lot of zip files and can't directly compact it because you're synchronizing, please contact our support ninjas by email (
omnifocus@omnigroup.com) or telephone (1-800-315-OMNI or +1 206-523-4152) and they can help you get your database down to a reasonable number of zip files again.
Hope this helps!