Mid-2012 MacBook Pro running REALLY slowly

Hi guys, I've recently been having a pretty big problem with my laptop. Basically, whenever I use almost any application for too long (5+ minutes) , the entire machine slows down to the point of ridiculousness. I checked the activity monitor, and it looks like kernel_task is using up some insane amounts of processing power - in the realm of hundreds of percents. I saw some similar issues when I did some research online, but none of them seemed to approach the numbers I saw on my machine. None of the solutions have worked, either.

Here's a list of things I've tried to resolve this issue:

• Resetting the laptop
• Resetting the PRAM
• Resetting the SMC
• Backing the laptop up, wiping the hard drive, reinstalling the OS and then restoring from backup

The problem still persists, and it's making the computer almost unusable. I've attached a screenshot of the activity monitor screen so you can see the issue - although I've seen the CPU usage go as high as 3,500%. I understand that it's normal for it to be using up large amounts of processing, but I feel like 8x the amount used by everything else on the system is a bit much.

Please help. I have no idea what else I can do to fix this issue. My laptop is a MacBook Pro (13-inch, Mid 2012) running Sierra 10.12.6.