For my system it hovers around 50-70ms[1][2]. Tested with an 800x600 window at ~7% cpu utilization on a Core i5. My desktop monitor is a Dell u2711, which seems to add about 15ms latency itself[3].
I'm not sure how much latency OSX or Chrome adds (at least a frame more than Firefox[4]); Mobile Safari seems to be a bit faster, as evident in the video. I don't have a second Windows machine for comparison.
Edit: When connecting on the same machine, latency is 2 frames (33ms) exactly[5].
