Try playing it again after you've closed LW down or immediately after a reboot. See if it's suffering the same problem. If yes, then it may be the file itself. Some people don't know how to deal with video conversion properly or use cheap ripping & conversion programs that aren't worth the time of day. So you might have been unlucky to get one of those.

However, the problem might be due to lag on your computer due to Java, particularly if LW's been open for a long time. VM (virtual memory) which OSX uses for all programs, tends to build up if LW's been running for a long time. This can slow your computer down a little. That's why I suggested not to jump to conclusions too quickly & instead, wait until the next time you reboot or ...

I forgot to ask, what format is the video? Is it a windows format like wmv, asf, asx, etc. ? Sometimes those can have difficulty playing properly, even within VLC.