Unity 2D on QEMU-KVM vs. real hardware

Unity 2D performance on an actual netbook with working Intel 945 graphics, using 256 MB of shared video memory: bad. Menus lag, window contents are slow to render, etc.

Unity 2D performance on a QEMU-KVM virtual machine, emulating an ancient Cirrus video card with 4 MB of video memory: excellent. There's no lag anywhere.

What's wrong here? Are the Intel graphics drivers that bad? Is shared video memory that much of a liability? Does KVM do something behind the scenes to buff up the emulated video card?

What gives?

 

What are the specs of the machine emulating?

 

4 GB RAM, Core 2 Duo processor. I've allocated 1 GB for the VM. Video card is a nVidia Quadro FX model, but when emulating a Cirrus card that shouldn't matter... Right?

Edit: I'll also note that the VM is a good deal more responsive than the host OS on the same machine. Maybe because it's reading from a smaller, contiguous area of the disk? Weird.