Since updating my large Mac to Monterey version 12.2.1 the very slow route calculation seems to have resolved itself. My love of BaseCamp is now rekindled, though MyRoute is still enticing me.
It had become very slow, sometimes taking a minute to connect roads just a couple of miles apart. It was one of the reasons I started to get involved with MyRoute, though the purchase of an XT sped that along, too.