Many thanks to you megavolt, it is indeed the performance setting that influences the refresh speed!!!
Curiously, of the 3 modes (power,balanced,economy), only the “balanced” mode drops the fps.
Another curiosity: in the control panel (top right), the power management button is illuminated only in “performance” and “economy” modes, and switched off in the problematic “balanced” mode.
One last thing: no more display lag or mouse movement.
I’m not going to close this post just yet, as I’m assuming that this is not normal behavior and that there must be some kind of setting to correct this problem.
If anyone can come up with a permanent solution, I’d love to hear about it!