If the whole server is lagging when several people play music and you are hosting the music off of the same machine or network connection as the game server, then you simply do not have enough server bandwidth to support both services at once. There is nothing I can do to reduce bandwidth use in this case but you could restrict MP3s/oggs to a low bitrate like 64 kbps. There is a setting in the settings.php file to enforce this.
There is a feature in Jukebox to have to the MP3 files hosted on another server and managed automatically via FTP. I used this myself once - the game server and website ran off of one machine and I rented another server space for really cheap that offered no PHP features and nothing but a lot of space and bandwidth, and set up Jukebox to manage it via FTP. In this case everyone still uploads to and uses the main website but Jukebox will quietly upload the songs to the FTP server whenever new ones are added. New additions are available immediately to play when first uploaded and then the server notes the change to the remote FTP server as each transfer completes.
If the "lag" you are referring to is a sudden jitter when someone starts a song, there is little I can do to improve that. It has to do with how the web browser is not threaded separately from the game. This jitter is particularly bad with the old media player used by the pre-Apple games but the new player is really light and has almost no effect.
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