Is your external display on the intel mac in scaled mode? MacOS has to do a lot less work if it is in its default display resolution. FWIW I use catalina on a

Is your external display on the intel mac in scaled mode? MacOS has to do a lot less work if it is in its default display resolution. FWIW I use catalina on a 2009 core 2 duo mac mini hooked up to a 1440p monitor in native mode, and the fan remains at idle when just clicking around the desktop.