Slow iMac only when the LCD is disconnected!

Here is a weird one… I have a faulty display on my iMac so I was thinking about hooking it up to a secondary display and make use of it. When I changed the hardrive to an ssd, I tried only using a secondary display just for testing and the computer was slow as a snail! I tried restarting it and i installed the macfans app to get the fan to work properly but nothing seemed to help. The only way to make it run as usual was to connect the screen. I did not have to connect the cable which provides light, only the data cable and the computer again worked on the new display… any ideas for why this is and or how to use it without having to have the lcd connected?