Friends of my father in law had a Mac that the wife used, her husband Gary used a PC. Until his PC died. So he got an account on the Mac and the first thing he did was to install „optimization software“, namely MacKeeper and a few others. Next I got a help call from his wife that the Mac was very slow, and her husband said he hasn’t done anything, but she doesn’t quite believe him - he likes to fiddle with things. So I removed the crap he installed and the Mac was back to his usual self.
Now several people with similar and even older machines have said they run fine and plenty fast enough. So it might be time to consider that it isn’t the Mac that is the problem, but something you have installed, possibly as part of the migration to the new machine. Which might also explain why it seems to have propagated since.
Btw MacKeeper is known Malware.