I find poor termination of RJ45 cables has some strange effects on NVR's. I have seen this issue out in the field and to fault find this I ran a couple of cameras connected to the NVR on the test bench with pre made cables no issues. Once I connected the pre-installed cameras back up, NVR menus were hanging and running slow. One by one I removed cameras until I found one camera had a poorly made cable which I replaced and the problems went away.