Improving the speed is most likely harder then compatibilty. It's the way programming works with everything, not just emulation.
You write the code as best you can, but you're always aiming for functionailty first. Once it works you go through and adjust the code to try and speed it up (if needed). In order to really optimize code well you sort of need to know every piece of it, and untill it's entirely written it's basicly impossible to do that. Of course they could go through and speed up what they have now, but it may end up getting undone when more features/compatability is added in.
Optimizating programs is one of the hardest things when programming today. To make matters worse most programmers are much worse at optimizing then they were 10 years ago. Mainly because for your average application it just doesn't matter, computers have gotten so much faster that it allows people to be sloppy and still have programs running at full speed. Of course in the case of dolphin they don't really have that play room with extra speed. That isn't to say dolphin is sloppy, it's just saying that it's hard to find people who are actually any good at it.