Long story short, I got a refurbished iMac Late 2015 from the apple store with 1TB fusion drive. Screen broke, Apple store wanted to charge me 700 euros, I said nah, bought it on China for 250 euros. Since I was already opening the iMac to change the screen I decided to upgrade the SSD. Bought a 1TB Crucial SSD. And the iMac was just extremely slow after I boot it. Fans weren't working either, they were at its max RPM. I can't explain you how slow the SSD was, it was taking around 10 seconds to open a folder. It was just unusable. Tried enabling TRIM, the fsck -fy command, resetting PRAM, all the articles I found online and nothing seemed to work. I installed the mechanical drive again, and it was also too slow. So I took it to a Apple certified store. Explained everything to the owner and he said, no worries, you just unmounted the Fusion drive and that's why it's too slow so I left the iMac with him, and after 2 days he calls me and says that it's a SMC board problem, 700 euros. Luckly 3 days after that he calls me again and says it's a screen sensor that is not working, and it will only cost 100 euros. Problem is that now I have an iMac with an SSD that runs ok, but not as fast as it should. Write speed is 227 mb/s and read speed is 316 mb/s. I checked and TRIM is already enabled, tried again the fsck -fy, reboot it, and now I'm not even getting 120 mb/s for Write speed. Any clue what can it be?