Why is low performance after changing screen and lvds cable?

Hello, I changed the screen of my iMac A1418 and the lvds cable, the screen turns on and the image appears, but after some seconds the main fan is a full speed and the performance of the graphics card is really bad!!!! is like the graphics card is working really slow

Hello, I am making more research and is not the graphics cards is the processor, is working a really slow frecuency!!!!, that happened to me when I tried to use the iMac without the screen connected with an external adapter before I replaced the screen!, please helpppp!!!