When I first jailbroke my iPhone 3G, I spent loads of time downloading cool looking themes via Winterboard and trying them out. Some themes were a real treat to look at, but there were stability problems (some themes seemed to slow the iPhone down more than others). Eventually, the novelty wore off and I went back to using the stock springboard theme.
Though I had stopped using themes, I didn’t actually delete winterboard from cydia. By this time, my iPhone felt sluggish and everything seemed slower than pre-jailbreak. That’s when I came across a post on iPhoneinCanada suggesting uninstalling Winterboard
to speed things up once again. I can’t say for sure how much faster my iPhone is now after deleting Winterboard, but I would say anywhere from 30%-50% faster! Needless to say, if you find your jailbroken iPhone slowing you down and can live without those shiny looking themes, try uninstalling Winterboard. Personally, I much prefer stability and speed to just looks. Give it a try and see if this works out for you.