It's just the way you look at it. You can take corn as an example of a high-speed emulator, but with a moderate to poor compatibility, but take Project64 as a pretty slow emulator with high compatiblity and stability. 1964 has the same, or probably even higher, compatibility as PJ64 and is almost two or three times as fast as Project64. Keep in mind that you can always do optimizations like adding more hacks, and better HLE (emulate less stuff), but keep in mind that there's always a chance that games get stuck or probably won't work at all when implementing this stuff.