Bit of a weird issue here and not sure how or why or even if its the correct diagnostic.

We use a video splitter to display the computers on screen and smart board. all the computers work fine except one that we replaced the video splitter with a new one.... the computer then started running really slow. so I replaced the computer with a spare until I can get chance to look at it. This new computer has started running slow.

Then the teacher brought in a laptop from home and connected it --- laptop ran slow... took laptop home and ran fine.

Bit weird?