Except that, based on the data you yourself provided, neither the CPU nor the GPU is busy enough for that to cause lag. Like I said, if it’s not CPU, nor GPU, then it’s most likely I/O. But hey, what do I know? I’ve just been programming for 20+ years, I’ll bow to your expertise 
And don’t worry, I’ll stop trying to help by asking you the question you don’t want to answer 