I've noticed that some people that lag with purpose...
Their shots are fast and their player runs fast, while you are slow and shots are loopy instead of pacy. Not to mention, on your own serve the lag is about 3 times lower than when they are on serve. In fact I've never had that issue with other people before.
It's quite annoying, played 2 people like that today and they lagged on bp's so they could tell your next move.