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Specs are:
i7 6700hq @ 2.6ghz (runs at 3.5 during gameplay according to task manager)
GTX 1070
16 gb DDR4 RAM
Windows 10
running the game at 4k max settings

On the initial launch, game was running at 60fps, but if I close and re-open the game during the same boot then the game will be locked at 39 or 40 FPS. I have this exact same issue with the Steam version and have already tried all the common fixes (drivers up-to-date, stutter remover, NVSE, etc.) but ended up with the same results. My GOG version is a fresh install and has the same problem as my Steam version with only 4 mods (FelloutNV, Stutter Remover, 4gb Patch, NVSE). If I reboot, the game will run at 60fps again, but only until I close it. If I re-open it during the same boot, like I said, the performance will drop to 39-40 and be locked there.

No one on the Steam forums has an answer, so I thought I'd try here.