Speed ? This emu can run very fast due to its dynarec !!!
But yes, its biggest lacks are sound and compatibility (MarioKart not playable ).
I'm quite wondering why NeonDS doesn't support sound. Sound is not so difficult to implement, even Dualis supports it. But NeonDS is also the youngest DS emu, and it's awesome that it can run commercial games at great speed.
I hope the developer will work on it because it is very long time. I know he have a life but open source would be nice for someone else to work on it if he wants to but I hope he is working on it secretly.