Aha, it's still a perfectly cromulent point. _every_ device starts off fast, phones laptops, whatever.
I've had quite a few MBs and they all slow down after a while, needing an overhaul/refresh in the same way that windows machines, phones etc do.
Get that red memory pressure and kernel desperately swapping pages in & out, woooo. Perhaps things are different with >16GB machines with the SSD mostly empty.