I think it depends on each one, MelonDS has a HORRIBLE performance on my PC while DeSmume I can run most games without a problem and with a good performance (and still with frame-skip) besides the possibility of playing online.
DeSmume and NO$GBA even runs on a potato while MelonDS requires a reasonably good PC and yet the performance may not be perfect.
Desmume can't even run on patatoes it's so bad