What is it that you are trying to play and how big is the file? I'm presuming your trying to play it on VLC or Media Player Classic via CCCP. Anyways, I think you should close as many open "user running" programs as possible. That should significantly speed up your computer and should reduce the lag. If not, I think you could set the VLC frame skip when you open your file. Try that and see if it works.
they released the new version of mplayer for windows with a user interface granted its still a bit buggy compared with 3rd party user interfaces. Also I believe the user interface version of mplayer uses more resources due to its bugginess but not 100% sure since I have a capable computer.
The top left is mplayer with the official user interface downloaded from the official website the name of the download from the official website is "MPlayer 1.0rc1 Windows GUI" which includes everything needed to play video except the extra mplayer only codecs which most likely won't be needed depending on what u want to play. The bottom right is a third party user interface which probably uses less resources and is not buggy at all.