The first problem is rendering within the guest. If you only have one GPU, then GVT-g [1] virtualizes it with just a bit of overhead. But it's Intel only.
The second problem is getting those pixels onto your screen in the host. SPICE is not as fast as Looking Glass [2], which sets up a shared memory buffer between the host and guest. This has acceptable performance even for modern games.
The OP doesn't seem to utilize these techniques, so I don't think it can plausibly claim to have the fastest configuration - at least not yet.