My version of gmplayer (0.90pre9-3.2) was sometimes very slow on playing mpeg movies (and only mpegs), while mplayer was running fine. The error message was:

         ************************************************
         **** Your system is too SLOW to play this!  ****
         ************************************************
!!! Possible reasons, problems, workarounds: 
[etc...]

The solution was to find the "cache = 4" line in ${HOME}/.mplayer/gui.conf, and change the value from 4 to 512. Everything is perfect now!
Maybe you could experiment with different values of cache, like 1024, or 2048.