How do I decrease my server's lag?

I'm running a Tekkit Bukkit Server using Hamachi on my laptop which has 16GB RAM.
Whenever one of my friends join my server they claim it has terrible lag and is basically unplayable. When I join the server I have no problem with lag. I'm wondering if this is a problem with upload speed from my internet or something else but I've looked everywhere and don't know how to fix it. Can someone PLEASE help me?

 

No, he can play singleplayer and other minecraft servers with no lag

 

Which processor do you have in your laptop? It takes a lot to run Tekkit compared to a normal server. When running the server, do you play on the server on the same laptop? What is your download/upload speed? You can check at http://speedtest.net/.

 

A lot of times, a laptop may not have enough RAM to run a Tekkit/Bukkit server AND still allow you to play on that or another server without lag if the server is running. Also, your internet provider may not have a good gbp/s which would cause lag to other people.
I would personally suggest to buy a cheap server to run what you want.
Check out this link for a cheap but quality server: bisecthosting.com