The thing is that any part of the game where there is a lot of detail (i.e. outside) will slow your computer down because it has a lot to load, whereas it runs faster in places like your house because there are less things for the emulator to load at one time. So it will be better and worse throughout the game......
You really need a computer with a lot of ram, and instead of No$GBA I would also recomend Ideas.....