Client side browser Lagging, not because of CPU

[SG]

This has been puzzling for a quite a while.

I check my server cpu usage and it never exceeds 30%, so I assume that it's not cpu. Also, my server memory never exceeds 30% either.

Now I can only suspect that their internet connection is slow~ or possibly their browser can't keep up w/ update on time.

Can someone help me how to pin point which side is lagging? Of course I myself never experienced such a lag. Please help, thank you!!!