I replaced the SSD in my MacBook Pro 15" mid-2015 with a 1TB Samsung 970 pro. Following the instructions here: the question about SSD replacements of MacBook Pro, it should work on PCIE 3.0, but now it only works on PCIE 1.0 with a significantly slower speed. Any idea why? I thought this would be a software problem, because I just removed this disk from another device where the disk works fine. Update:
It becomes normal after I enable TRIM and reboot, while I'm still not sure which one of them works here.