In a brand new save:
In-game, my reported UPS in fact hit near the expected 3840 with quite a bit of fluctuation, while FPS remained at a stable 60. (I do not have the vsync options turned on, but my monitor's refresh rate makes a higher-than-60 FPS wasted anyways.)... but moments later the the UPS promptly crashed down into the low hundreds with a lot of fluctuation.
I thought this was odd and unusual compared to what I've been able to observe in the past, so I alt-tabbed to Discord (on a second monitor, so Factorio was still fully visible). FPS immediately skyrocketed into the thousands(!), and UPS returned to the expected around-3840. Alt-tabbed back into the game and it was immediately back to before -- 60 FPS, low hundreds UPS.
What I've found so far is:
- Alt-tabbing to anything restored full UPS/unlimited FPS -- even before switching tasks (i.e. the Windows 10 window showing what you're alt-tabbing to alone was enough to fix it
- Simply clicking on Discord did not trigger this effect. Clicking on a Chrome window, however, did.
- Sometimes the game would run at full 64x speed while focused, for awhile. This usually ended when autosave triggered (which is quite often when on a 5 minute interval that's sped up 64x) and usually would not recover.
Update times in show-detailed-info seem to be consistent regardless of the current reported UPS, but the slowdown is definitely real (i.e. the reported UPS is correct), as evidenced by noticeable changes in day length and cloud movement speed.