For anyone else with this problem, here's the culprit: PRIVATE RELAY (under iCloud, in Settings) — TURN IT OFF! Apple's VPN slows your internet speeds down to dysfunctional. I went through six different AppleCare Advisors (including three Senior Advisors), a trip to the "Genius" Bar, and sent my MacBook Air to Texas for repair—all to no avail. Finally, one very astute Senior Advisor fixed the problem in five minutes when my computer came back from Texas, still with abysmally slow internet speeds. I also had the same slow internet speed problem with my iPhone. Once I turned off Private Relay on my iPhone and MacBook Air, both devices deliver screaming internet speed. The probable reason why the "factory reset" worked was that Private Relay did not get turned back on.
I swear, Apple Care and the "Genius" Bar in general get less and less competent every six months or so. It's pure luck of the draw that you get an advisor who actually knows what he/she is talking about—and unfortunately, they are a rare breed.