As far as I recall, DS emulators didn't have ARM recompilers before DraStic, they were all interpreted which is painfully slow. Exophase wrote an ARM recompiler which gave the initial huge boost to speed, and then it was tweaks and cleverness form then on to improve performance further, little by little.
Dolphin, on the other hand, does have an ARM recompiler, there's no sudden burst of speed to be gained: it's already into the "tweaks and cleverness" which usually only gives little bits of speed boost here and there.
The two emulators are in different situations, what happened with DraStic won't happen with Dolphin. That's not to say there isn't room for improvement in Dolphin, it'll just be slow going, shaving percentages of CPU usage.
