I clearly remember old times with Windows 98/ME, where if you keep moving mouse, long-running operations like directory coping would be faster, thanks to CPU power savings not kicking-in.
In the Windows NT series (4, 2000, XP, etc.) the sheduler gives the forground application an higher priority. It runs a longer CPU time than all other applications. This was probably also similar with Win 3 and 9x series but probably with many hacks and glue code on top of DOS.
Moving the mouse also prevented the screensaver, an application that would "save" the screen by drawing fancy graphics to prevent static phosphore burns on CRT monitor and slowed down the system
If I remember correctly, one of the first moves towards power-saving in the stock-OS was Win 95/98/ME (one of those) to start using the HLT instruction (stop processor up to next interrupt) in the main OS, was a busy loop until then.
Notebook manufacturers (I worked with machines from Toshiba and Compaq in the 90s...) provided DOS TSRs and windows applications and drivers to control the CPU speed and display brightness.