My iMac, bought in 2021 has a thin blue line down center of screen and performance has slowed drastically

Hello -- the original poster was using a 2013 iMac so your situation sounds very different. For that reason, you should start a new post describing your problem and providing information about the year and model of your Mac. The best way to do that is to run Etrecheck ( free download) and post the output here as the other poster did.

The fact that you have a blue line down the center of your screen suggests some hardware failure as well. If nothing shows up in Etrecheck's output you would be best served by taking your Mac to an Apple Store or Apple Authorized Service Provider for diagnosis and service. If you still have AppleCare repairs may be covered.

Download and run Etrecheck. 

Copy the report

and use the Additional Text button to paste the report in your reply.

Then we can examine the report and see if we can determine the cause of the problem.

Download and run Etrecheck. 

Copy the report

and use the Additional Text button to paste the report in your reply.

Then we can examine the report and see if we can determine the cause of the problem.