I play games like Skyrim and Far Cry 3 without much issue, but when I put in Paper Mario: Thousand Year Door or Animal Crossing the game cannot run a constant 50 FPS. Why is this?

My PC specs:
AMD Athlon(tm) II X2 250 Processor
CPU Speed: 3.0 GHz
RAM: 3.1 GB
Video Card: ATI Radeon HD 4600 Series
Sound Card: TASCAM US-122L

Now is it the emulator or is it the fact that my PC does not have the power to play GC games on it.

I appreciate the help.