It's solved! I have to share this. The last few months my macbook pro have been getting slower and slower. I have not been able to use an external monitor without some lag and if I throw on a video on youtube/stream/what ever it nearly collapses. Kernel_Task (under processes) is going through the roof and I've heard theories of too many USB, old hardware yada yada. I tried a ton of different solution tips (rebooting, reseting nvram etc), but nothing seemed to fix it. Somewhere today though, I found a tip about cleaning out the fans inside your computer. This was actually fairly easy, but I guess letting a pro doing it is smartest. I myself bought a 'pentalobe screwdriver' with 1.2mm for opening it up, then I used compressed air spray to clean out the fans. Now, 20 minutes later, I'm back on 2 external (2!) monitors, doing work on one and watching HD youtube on the other. No lag. Mac is more silent. Kernel_task hovering around 30%. Finally.
TL;DR Clean the fans from dust. Youtube how to.
TL;DR Clean the fans from dust. Youtube how to.