Ok, after some testing and researching I can say for sure, that on my system the framerate appears to be capped at 60-62 fps, no matter if windowed or not. If tabbed out or unfocused it is 20 fps. This is behavior is somewhat expected, after looking at the changelog for Alpha 0.70:
Quotes:
- Enabled VSync for “Good” and “Fantastic” quality settings, and set the target frame rate to 60. This should decrease the game’s performance hit on fast computers.
- When tabbing out or unfocusing the game, VSync is disabled and the target frame rate is set to 20. This should make the game use significantly less resources when not in use.
However, this changelog is over 2 years old and the mentioned quality-options don't even exist anymore. Also just some days ago someone reports Scrolls running at 120 fps here (albeit on Linux, which is unsupported atm), so it might just be some sort of forced Vsync (I have a 60 Hz monitor).
I don't want to add wrong information, so I leave "Vsync" and "High frame rate" at "unknown" for now. Additional help and testing would be appreciated.