Downgrading to 2.2. will not solve the issue of lag. The general solution to lag issues is in finding out what program is causing the lag and uninstalling it or not using it. In this case, the source of your lag is the Stage UI itself.
Use an alternative launcher (ADW EX, LauncherPro, or Go Launcher EX) in place of the Stage UI and you'll see an improvement in both speed and battery life. Reason why? The Stage UI uses a lot of CPU cycles to do its job. As a result of this, the battery is drained much faster than it should be. The source of the lag is that the memory is used inefficiently by the Stage UI.
If you want to improve your battery life even further, go into Settings>Location & Security Settings and uncheck the "use GPS" section. Doing this will prevent you from using Google Navigation unless you turn the GPS antenna on, but the results are worth it. To have easy access to the GPS function, use the included Power Control widget on your new launcher.
Obviously, I have a Streak or I wouldn't be posting here, but with these two changes to the Streak I have seen my battery life improve tremendously. Currently, my phone is sitting at about 65% of full charge, and I've had it off charge since 5am this morning.