2 macs sharing external hard drives

I successfully have my mac mini's external hard drives showing up on my macbook pro, however I have run into into a problem. When bringing up programs like i tunes I get get a very slow response or application not responding, I songs and videos show up in itunes after hours upon hours of loading them in. In Plex t also has this problem. It is hit and miss when I try to lay a video that is on the hard drive. This is only happens with video and music. Other files I have no problem. I am using an apple extreme router and an apple express router as a booster. Any ideas on how to fix this?