I have an existing Moodle 3.6.4 site with boost_campus theme and theme designer turned on the layout breaks. I've seen other people report this but not much in the way of a definitive reason or fix.
I changed my php's script timeout from it's default (which was 30 seconds) and now the theme designer loads, but it takes a LONG time... in the profiler I see that it's taking 1.4 minutes to render the scss file - other theme files seem to take much smaller amounts of time (still pretty slow though - 14 seconds for a 575 i file? What, Moodle?)
Any ideas what I can do, other than pressing "Purge Caches" every time I need to refresh?