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.
