Fallout 4 can play UHD medium but can't capture 720p low

[DynV]

I don't have the best system but I can play Fallout 4 at UHD medium settings + antialiasing off & anisotropic off (they're at TAA & 16 samples correspondingly at that default setting) with little-to-no tearing but can't capture 720p low + antialiasing off (it's at FXAA at that default setting) at even 10 FPS; you can see for yourself 3 attempts

I'm not a difficult gamer, I played Quake 2 at 320 x 180 for at least a year, so 10 FPS is fine by me; I suspect OBS is at fault since I have at least that when I'm doing the 720p capturing (with my eyes during that time, not when looking at the output file).

I'd appreciate help