A question on the screen x1, x2 etc

[Mugetzu]

Hi, and noticed that the speed is reduced when I select the screen x2

example FF type-0 50fps(1x) but when I use screen goes 35-40 (x2)

any fix for this????

Thanks

NVIDIA GeForce 6150SE nForce 430 (512 MB​​)

AMD Sempron 2.2ghz

2GB RAM