I hope this ends more optimized than DS emulators. I can barely run Mario Kart or NSMB at a good framerate at the original resolution. Meanwhile I can play everything i've tried on PPSSPP at double resolution and full speed.
Most likely because you're using software mode in whatever DS emulator you're using, which means all of the rendering is being done by your CPU. There are options for GPU-accelerated rendering like DeSmuME X432R.