Chosen Solution
I have exactly the same problem with MBP 2015 retina 15” very slow . The cpu fans run load though from time to time. Sometimes after a reinstall or fresh install it works for a few days perfectly but same slowness comes back after a while consistently. Ssd wasn’t the problem as I used an external bootdrive to boot Macos from also replaced with another ssd. Original & high speed ssd . ( sintech) I wonder if it has to do with the graphics card as every times it runs well for few days and I install imovie , adobe IA or mpeg driver for vlc it started to be slower. Could it be the graphics card?
I would run the onboard diagnostics to see what errors where popping. Restart your system and the press the D key to enter. Let us know what you set for errors. Referece Mac startup key combinations
