I spoke too soon. Not enough investigation by me...
If I right- or command- click a callsign in the Spot window and then click on QSY, it takes about five seconds for the KX3 to do anything and the beach ball runs for part of the time.
I don't think it's a driver problem, because moving the VFO by clicking on a digit of the frequency in the control panel works instantly. Other buttons work, too.