I had something similar when I first started ripping 4K. What I found was the software was trying to rip a disc bigger than expected, so was trying to compress on the fly rather than make a 1 to 1 copy.
I set up the copy function as BD100.
Once the disc has been read and is ready to copy click on the spanner icon and make sure the disc size is bigger than what was read.
I don't have my drives to hand to take screenshots to show what I mean, but I hope what I've described is what you're experiencing and a way to fix it!
