Chosen Solution
So i bought a used macbook air, got it sent from the previous owner. Have been advised about high fan speed issue, and a solution- mac fan speed program, that solved it. So i got a working laptop with high sierra. Since it is a used model i thought i might give it a clean install just to be sure. Started the install from USB like i did many times. Took about an hour and when the system was up and running again to my utter surprise it has gotten painfully slow… Stuck here, have no idea what to do. Since it was working fine earlier today as it arrived, I assume it’s a software issue/user error on my behalf!? Additional info: tried SMC reset, task manager doesn’t show any cpu hogging process. Before i reinstalled there were 2 partitions that i merged, its a 128 GB SSD model. Tried AFTP and mac os extended format before reinstall, doesn’t seem to make a difference. When i was starting the boot option tool there was a windows option shown as well, so i guess the previous owner was a bit of tinkerer :( Appreciate all the help!
Your high fan spin already suggests a sensor problem usually from a liquid spill. First try to boot without the battery ( problems with the batt can cause slowing your mac) if is still slow folow this tutorial to remove the logic board and inspect it for any traces of corrosion If you find corossion first of all take a photo for later use and then try and clean it with alcohol (not brandy or wiskey but pure alcohol 90 degrees) if you are lucky maybe this could solve your problem:
