Re: Sims 4 causes severe system lag (not in-game lag)

This evening, I installed Catalina (10.15.3) on an external SSD to test Sims 4.  Origin was freshly installed on the external, and a clean game folder spawned, but Origin recognized the Sims 4 program files on the internal drive and ran the game from there.  Specs of my Mac (the same as the first one Bluebellflora listed, other than a slightly weaker CPU):

MacBook Pro 15", 2017

Intel i7-7820HQ

16 GB 2133 MHz LPDDR3

AMD Radeon Pro 560

Intel HD Graphics 630

512 GB SSD, ~65 GB free

the external has ~180 GB free

I only own a handful of packs: EPs are Get Famous, Island Living, and Discover University; GPs are Strangerville and Realm of Magic; SPs are Moschino and Tiny Living.

I tested on both the default high and ultra settings, as well as with edge smoothing off and high, and post processing and uncompressed sim textures off and on.  I did not see any system lag when opening or switching apps via the dock or cmd-tab, even with two browsers and several other apps open.  I also didn't see any corresponding fps drops in-game.  However, when I clicked any of the dropdown menus across the top left of the screen, the Sims 4 screen itself froze in place.  But the game clock continued to run, and sounds continued to play out as normal.  When I closed the menu, the screen jumped ahead to reflect the elapsed in-game time.  There was no lag in the responsiveness of the system.

I also tested with another app in the foreground and Sims 4 open in the background.  In that case, opening a dropdown menu had no effect on the game, which continued to run unpaused regardless.

I also see the same lines across objects, again in high (first image) or ultra (second image) settings.