You didn't notice this issue when you had the Insire on Froyo?
GB should work fine on a 1Ghz phone as it was designed for it. The Nexus S, which is what Google used as the testing device for GB has only a 1Ghz single core processor. Have you tried to clear all of the cache after you installed GB?
Sign up with Dropbox using my referral and get an additional 1/2GB on top of the 2GB you get for signing up http://db.tt/YbULMZX