Xephyr: mouse does not move smoothly

I am using xserver-xephyr in Debian Wheezy. My mouse inside the Xephyr window does not move smoothly. It looks as if the refresh rate was very low. When I move my mouse across the screen, it maybe refreshes 3 times. This looks more like the cursor was jumping, rather than moving smoothly.

Is it there some configuration I can change to make the mouse cursor move as smooth as normally ?