Vincent - it sounds like you've taken most of the usual steps, but I wanted to mention one thing in case it helps. Is Auto-Sync turned on in the Sync preferences?
If not, it may be worth experimenting with. You'll need to tap the sync button from time to time, of course, but it should reduce your launch times considerably. I just make a habit to tap the sync button when I'm done with OmniFocus. Stick the phone in my pocket and let it sync while I'm otherwise engaged.
If you have auto-sync turned off, those launch times are longer than we'd expect; if you contact the
support ninjas, we may be able to figure out what's going on.