>Mmmmm I thought it was that the task actually on the display got a
>boost by the scheduler, making the interactive performance quicker at
>the expense of background tasks.

SunOS 3.x used to do this for SunWindows.  The kernel kept track of
which /dev/winXX device the mouse pointed into, and it would fiddle
with the effective "nice" level of a process associated with that
window.

Just FYI,

Chris