I've lowered the VM to 2 cores, but the problem persists. The only (expected) change is that now when the windows machine "idles" it takes half the CPU time as before. Other than that, the behavior is exactly the same;
CPU spikes for the VM until the second I bring the VM into focus, at which point it drops to normal. According to the VM (windows task manager with all processes displayed) it's been at ~1% CPU the whole time.