What did you use to jailbreak? Initially, I used jailbreakme on a VZ iPhone 4 running OS 4.2.8 and it was completely buggy. Basically, unusable. They say it's compatible but it's not. I had to restore and do it over again with pwnage tool and now it works fine. I'm running some Winterboard stuff and my phone really hasn't slowed down whatsoever.