Not sure. I use mplayer....
If it's "slow" (not sure what you mean by slow, I assume high-cpu playback), you need to configure mplayer a little bit.
Mplayer has a veriaty of outputing modes. You can explore them with the command line version.
mplayer -vo help
That will tell you what video modes it supports on your computer, and
mplayer -ao help
will tell you what audio modes it supports on your computer.
x11 is default, I think, but it's pretty slow when it comes to resizing output. I use "xv" because my nvidia drivers handle that with very low cpu usage. For sound I use alsa.
For configureation you have a system wide configure in /etc/mplayer/ (or something like that.). You can edit that, but for a custom user setup copy the mplayer.conf to ~/.mplayer/config and edit that. That way you can play around with the different options like filters and stuff like that to get the best quality output possible. Mplayer is the highest quality player anywere, but it's mostly for movie buffs. Xine is easier to play around with.
But I agree for a plug-in it's sub-optimal.