My guess would be GPU load or GPU/PCI-E bandwidth. You could check GPU-Z to verify that your graphics card is running at maximum PCI-E speed. Based on the profiler information in the log, it looks like the problem is downloading the rendered frame from the GPU to RAM for the encoder (20ms). At 60 FPS, the whole frame has to be rendered and delivered to the encoder in 16.6ms. Lowering your OBS/encoder FPS to 30 will increase that window to 33.3ms.
I doubt it has to do with not having enough RAM, though RAM speed could be an issue. If the GPU is not receiving enough power to run at maximum performance levels, that could possibly affect things.
Anyone who has more knowledge to offer is welcome to chime in, especially if I'm mistaken.