It's never happened before, but as of recently, one of my players tried to track their token's movement (the one where you press/hold "Q" as you move the Token) and it caused my browser (and apparently the other players' browsers) to stall immensely, freezing up in huge gaps and even crashing.
I tested it again on my end and the same thing. We did it a 3rd time with a different player and same thing. So we pretty much had to stop using it entirely, which sucks because it's never done that before in our games and we figured to let Roll20 know it seems like some kind of bug maybe causing it.