If you're talking about Pokemon, then that's because the Overworld areas use the DS's 3D graphics engine, while the battles don't. 3D requires significantly more power to emulate. Again, make sure you've got the very latest version of no$gba (you'll have to search Google for it, since you can no longer pay for it to get it legitimately). The latest version has a special 3D rendering option (enabled by default I believe) that is a good amount faster than the other options, though it may have slight graphical glitches as a result. Give it a shot at least.