High CPU load (D3100 Ubuntu 15.04)

[znd0]

I get an extremely high FPS reported from glxgears:

$ glxgears 
Running synchronized to the vertical refresh.  The framerate should be
approximately the same as the monitor refresh rate.
303 frames in 5.0 seconds = 60.553 FPS
1203 frames in 5.0 seconds = 240.384 FPS
8173 frames in 5.0 seconds = 1634.242 FPS
9915 frames in 5.0 seconds = 1982.991 FPS
8718 frames in 5.0 seconds = 1743.354 FPS
8002 frames in 5.0 seconds = 1600.181 FPS
8554 frames in 5.0 seconds = 1710.762 FPS

However, it LAGS

I'm running xfce...