[1.1.19] Factorio slows down by 50% if focused

[Kiu]

Very weird effect in current version. Most noticeable in freeplay, FPS/UPS drop about 50% almost immediately after starting game.
Didn't have the problem in the previous version (experimental).
Most weird part is that the FPS/UPS drop disappears as soon as you put another application in front of the Factorio window.
Opening menu/pause doesn't make any difference. Rebooted PC and tried directly after login.

EDIT: even happens in "new game"

[Rseding91]

Every time Iv'e seen or heard of this it's always on 4k screens. Try exiting fullscreen and then re-entering it (alt + enter)

[Kiu]

If i go full screen the UPS/FPS go up immediately (to the usual 60/60), as soon as I switch it back to windowed it drops down again. But not instantly, its more like slowing down over 2-4s.

[NotRexButCaesar]

I wonder; do you have v-sync disabled or enabled?

[Kiu]

Tried with and without, no difference (InGame-Setting).

NVidia G-Sync is enabled, but nothing was changed there in the last months. No NVidia game profile exists.

No driver updates or things like that done in the last weeks - nothing except (maybe) some Windoze updates (Win EventLog says 'nothing happend').