Port forwarding probleme minecraft server lag

hey i port forward my minecraft server and it work but my frend complain about lag ay idea on how to fix that

Have you allowed it access to more than the 1gb preset amount of ram? If your computer is up for it, give minecraft some more memory. If your using the Jar version Open up notepad and copy/paste this line in there: java -Xmx2048MB -Xms2048MB -jar bukkit.jar.  You can change the ram to what you wish. I either go to 3gb or 2gb.  That line will give the server 2gb. Aslo save the file as "Start.bat"