Festival tells you how far off you are in milliseconds, and gives you extra points for perfect timing.

But that's still not an across the wire latency.

The “perfect” timing window in Festival is +/-5ms from gem position (10ms total afaik). Harmonix has always been exceptional with their beatmatching code. There are physical limits to the black magic of latency compensation, however. If your input has to travel over the wire before the game can see it, then it’s unavoidable without heavy compensation