So there are three types of lags that may occur while playing ZK (or any Spring game):
1. CPU not fast enough to process all the game mechanics (you have 100% CPU load)
2. Graphic card unable to redraw all the stuff many times a second (you have low FPS)
3. Network problems (you are lagging with high ping even though your CPU is under 100%)
This last problem may be caused by so-called bufferbloat. It may happen even on the high-speed internet connection.
It causes poor performance for voice and video chat, online games to lag, and generally makes people say, “the Internet is not responsive.”
is a good article about it, with possible solutions. Or just look up "bufferbloat".
To make sure you don't have it, you need to get grade A in this test