Game speed is determined by CPU architecture and clock speed. The GPU is not that important. Remember that it is an emulator and NOT a PC game. An i5 (first or second generation) with 3+ GHz should be enough for a lot of games but not all! An overclocked i5 2500K is needed for the more demanding games.
most emulators are resource heavy because they have to simulate all the chips and buses accurately for the emulation to funtion correctly, I was suprised at how much cpu was needed just to emulate an Amiga 1200 ((WinUAE) Motorola 68EC020 at 14MHz) it was near enough equivelent to what was needed for the Wii emulator ((Dolphine)PowerPC-based "Broadway" at 729 MHz)