Did you have your feeds (rss, twitter, facebook) set to refresh every time you pulled down ISX? I changed all mine to only update every 6 hours (essentially manually then) and my battery is excellent on iOS 5 with ISX installed.
To diable auto refresh just go into the ISX settings and into each plugin and switch refresh settings for "on ISX view" to whatever time you want (mine, as i said, is 6 hours.. that way it updates only when I want to refresh it)
I tried your advices but still having lags, the woerd thing is that it only happens after respringing, the second time is smooth.