!important has its place. Trust me on that one. ItIt's saved me many times and is often more useful as a short-term solution, before a longer and more elegant method to your problem can be found.
However, like most things, it's been abused, but there's no need to worry about 'performance'. I'll bet one small 1x1 GIFGIF has more of a performance hit on a web page than !important would.
If you want to optimize your pages, there are many more !important routes to take ;) ;)