Many mice support report rates of up to 1000 Hz. While this is great for gaming, it is horrible when used with Synergy. It causes so much lag because the network cannot keep up with all the mouse events. I had to reduce my mouse report rate to 250 Hz, and that still lags if I use Synergy over WIFI (it's fine over ethernet).
It would be nice to have an option in Synergy to merge/reduce the mouse events being passed between machines. For example, combine all the mouse move events within a certain time period and send out a single mouse mouse event every x milliseconds (e.g. 8 ms for 125 Hz). This will reduce network traffic while maintaining responsive mouse input for the local machine.