Comment 9 for bug 1880405

I hope this helps: I see these errors in my syslog:

Nov 4 10:43:32 parse2 gnome-shell[4026701]: JS ERROR: TypeError: windowActor is null#012_addWindowEffect@resource:///org/gnome/shell/ui/closeDialog.js:90:28#012vfunc_show@resource:///org/gnome/shell/ui/closeDialog.js:162:14

Nov 4 11:17:38 parse2 gnome-shell[4026701]: message repeated 34072 times: [ JS ERROR: TypeError: null has no properties#012_onFocusChanged@resource:///org/gnome/shell/ui/closeDialog.js:135:9]

Nov 4 10:51:50 parse2 gnome-shell[11582]: [11584:11584:1104/105150.374036:ERROR:CONSOLE(1)] "[Shields]: Can't request shields panel data. Error: No tab url specified", source: chrome-extension://mnojpmjdmbbfme
jpflffifhffcmidifd/out/brave_extension_background.bundle.js (1)

Yet More Info:
While it was running slow, I did a gcore, and it showed a single active thread (90% CPU) in the garbage collector. That may have nothing to do with this, but who knows...

gnome-shell: 3.36.4
Ubuntu 20.04
Hardware: 64g memory, 1Tb NVME drive, Asus ROG Strix GeForce RTX 2060, 4 Monitors
          Cores: 16 Threads: 32 AMD Ryzen 3950X
browser1 = brave tabs = 67
browser2 = firefox tabs= 64
various other windows Quake, rocketchat, Thunderbird, MATE terminal