I suspect it is a KVM issue, as I see the exact same lag-effect on Windows 7 installs, comparing vBox and KVM. I have not actually tried full-screen playback on either Whonix or Windows via KVM, as basic interaction with the desktop is still a question. I did push the vBox Video Memory field up to 128 MB, and I it runs either OS very well. However, I don't understand what that vBox setting actually corresponds to in the KVM configuration (i.e., vram, vgamem ...?).
I might add, this same machine offers good performance running Qubes OS, so I am a bit frustrated that I cannot get KVM up to speed.