The problem a friend was occuring was, that on a fresh Windows install settings like cl_refresh_rate are set to 480.
Since the render calls take longer time than no render calls, it happened that client was sleeping on one update cycle and then took to long in another, where it called the render functions, leeding to less FPS than the cl_refresh_rate. this caused mouse lags and rarely frame drops.
another thing is, that select on Windows is non microsecond accurate, or just often returns ...