Windows – Does having Windows Aero enabled negatively impact gaming performance

In our testing we've seen that turning on Aero consumes only about 1-4% more of battery life. In terms of making your battery last longer, turning off Aero will not go very far while at the same time costing you some of the cool features that make Windows Vista fun to use, such as Flip 3D, taskbar previews, window transparency and so on.

Taken from here

There is a good reason for this. Aero uses a lot of alpha blending, which needs hardware acceleration to be performant. Even without a dedicated graphics card, the onboard graphics can process the layers much much quicker. Unfortunately, what that means is that the screen buffer gets handed over to the graphics chip, and the OS never sees the final rendered frames. If they're just being sent to the monitor, that's fine, but Aero depends to a certain extent on hardware acceleration. When VNC tries to run with Aero, it will have to kill some of the hardware acceleration and force the CPU to do rendering, so it can capture the screen state. This unfortunately can force Aero out.

I work for RealVNC, and we use a mirror driver to help address this somewhat by grabbing the draw commands before they're sent to the chip for compositing, and hold our own buffer. I know that UltraVNC uses a mirror driver too in some way, but I've not looked into their implementation. You could try ours! I suspect if the performance is just too low, there might not be a way to force Windows to do compositing and blending on the CPU.