...try to change the "Default Integration Method", either from control panel or by specifying the method inside the schematic, by adding a spice directive like this:
.options method=gear
Fascinating. Changing from "modified trap" to Gear resulted in the simulation speed going back to the great speed I had when using MacOS High Sierra! The main downside is that it doesn't fix UI slowness when dragging inside the waveform to Zoom. But simulation is significantly faster while using Marching Waveforms.
I must assume that Gear is less accurate because it's so much faster? When I zoom into the ripple on the voltage output of my simulated switcher, the sawtooth is less consistent with Gear than with "modified trap."
Very interesting! Thank you for the suggestion.
However, the UI slowness is still there. In the early part of the simulation, with Marching Waveforms ON, clicking on a menu takes 2 seconds for it to drop down. But at about the 18ms point in the simulation, it takes 40 seconds for menus to drop down! Then after the simulation is done, clicking to drag within the plot window is very, very slow. So while using Gear helps, it's not a solution for the root problem.