That's odd. Can you try the 4.0 or 3.5 master build on both?
Your GTS 250 is capable of handling some upscaling (around 2x w/ some AA), so its not an issue. Older revisions of Dolphin will run better on Windows XP because that was what it was optimized to, so you will likely see the better performance on older revisions under XP. Newer versions sacrifice some speed for better accuracy, and have been optimized for newer instruction sets (AVX, SSEE 4, etc) that your CPU doesn't have, and are more optimized for that precision under Win 7.
On Win 7, make sure you're using high performance mode under Power Management (I'm not sure if that will have any effect, though.)
Also, try using OpenGL, and enabling Vertex Streaming under the game's properties.
P.S.: Do you have OpenCL turned off and OpenMP turned on under hacks?
Your GTS 250 is capable of handling some upscaling (around 2x w/ some AA), so its not an issue. Older revisions of Dolphin will run better on Windows XP because that was what it was optimized to, so you will likely see the better performance on older revisions under XP. Newer versions sacrifice some speed for better accuracy, and have been optimized for newer instruction sets (AVX, SSEE 4, etc) that your CPU doesn't have, and are more optimized for that precision under Win 7.
On Win 7, make sure you're using high performance mode under Power Management (I'm not sure if that will have any effect, though.)
Also, try using OpenGL, and enabling Vertex Streaming under the game's properties.
P.S.: Do you have OpenCL turned off and OpenMP turned on under hacks?