All of them
Today's computers are fast, but far from what's enough. There're many many situations where performance is critical - if you do streaming media servers.
You customer might have a big disk, let's say, 1Tb. Which can be taken up by 1000 HD movies, if you want to make it a service it's far from enough, isn't it?
Well I'm not sure if this count as “optimization", what I do is I use Java instead of C++, and the development get 10 times faster. I feel like I'm telling what I think directly to the computer, very straight forward and totally rocks!
BTW I believe to speed up development your development process you should choose java, never try rubbishes like python... which claims they can shorten you DEV time.