"Why the realtime preview is frame skip?"
1.Please login the camera from IE to test whether it is the reason of camera.
2.Please check all the channel in NVSS to make sure whether it is caused by switch you use. Fistly, check the switch which connect with NVSS and make sure that it is a 1000M switch. Secondly,we need to think about the processing capacity of the switch. For example,you have 128 cameras which connect with NVSS,each of them sets the main stream is 3Mbps,then you can figure out the total instant value should be 384Mbps, but it is far from 384Mbps when you check 'Camera Data Stream Statistics' in NVSS,it may be 200Mbps.then we have reasons to doubt processing capacity of the switch, it caused dropped data packet seriously. you should change a switch to have a test.
3.If there is only one or two cameras have the problem,then check the brand of your cameras.it May be caused by the cameras and NVSS software compatibility issues.