Windowed (Fullscreen) Game lag

In Windowed Fullscreen mode the game lags immensely, including the UI. It is as though the speed is at the lowest setting, though it is not. Changing the display option to “Fullscreen” or standard “Windowed” results in the lag being gone; if I switch back to Windowed Fullscreen, though, the lag returns.

Is this a fixable issue?