Nobody actually uses OmniFocus. Nobody actually owns it. There are some folk who haven’t heard of it, there are many who are on PCs and so can’t get it, but then the rest of us live in OmniFocus and it owns us. Not the other way around. I live in this application and I for one welcome our new OmniFocus overlords, but still I have never once done this trick for speeding it up.

Truth be told, it’s never got all that slow for me. But the time will come and this is a smart guide from a very good new site called The Sweet Setup that tells you why you should archive off your old tasks and how it will help you:

How to archive completed tasks in OmniFocus for faster syncing