EDIT: it should be: "linux guest on a windows host)?"
Host OS must not make the difference... But host OS load may make the difference.
To have max speed you must move to a bare metal solution like ESXi.
Wouldn't the speed with ESXi be limited by the spec of the (5-year-old) controlling computer? Or high definition flash video is played at full frame rate no matter what the computer you see the user interface on?