My hikvision 64ch is lagging after adding 64cam on it

My hikvision 64ch Nvr is lagging after adding 64 cam
Have you checked the specifications of the NVR to see if you are either exceeding or at the upper limit of its rated bandwidth for the number and resolution of the cameras connected ?
 
As Tony alluded to..... NVRs have a maximum number of connections, but they also have a maximum bandwidth they support. An NVR is really just a self contained computer with a CPU, and the more bandwidth you are using the more CPU power it uses. Eventually you can run out of CPU power and the system will begin the get laggy. It sure sounds like you have just reached the upper limit of that NVR.
 
+1 above- just because it is 64 channel doesn't mean it can handle 64 cams at higher MP and max bit rate and FPS.

Many people need multiple NVRs.

You may have to drop FPS, bitrate and resolution on some cams to make all 64 work.