Timing out on my OWN Minecraft server?

I'm hosting a Minecraft server on a second computer in my home. It runs without lag and all of my friends can join it. But whenever I join it on my main computer I "time out" or there's TONS of server lag. Not FPS lag. Can someone help me!

You need better internet, you're hosting a server and playing on it meaning you're using more bandwidth than if you were just playing on another server.