Bad performance when using radeon gpu

Very low fps when i'm using radeon gpu compared to older nvidia.

System :
Latest stable (non beta) drivers with both cards.
Phenom 2 x6 @ 3.4ghz , 8gb ram, Radeon 7970 ghz edition : 3gb of vram.
Fps depending on settings anywhere between 23 - 88 without v-snyc.

Same system with Nvidia geforce gtx 260 : 896mb of vram.
Fps depending on settings anywhere between 53 - 212 without v-sync.

When using snapshot i had max fps set to unlimited.

When creating new world fps drops to 3-10 using radeon and 30-40 using geforce.