Oh I think I might have found why it is so slow:
Somehow in this file it managed to get the Grid to extents of 100.000 even though the maximum is 10.000.
On my fast machine:
Grid on = 17 fps
Grid off = 60 fps
On the slow machine:
Grid on = 0.5 fps
Grid off = 5 fps
@wim Do you know if the Grid is somehow optimised for fast rendering or is it literally just drawing 200.000 lines in the case above?