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.

Update after some playing around and testing further:

I have all setting on the highest possible because I like my game to pop. If I change the 

RenderShadowsEnabled

value to false in the GraphicsRules.sgr file it instantly fixes the problems:

• My system doesn't lag
• I can happily tab between apps whilst the game is open
• FPS jumps to 50-60
• Overall gameplay is a lot smoother
• The awful lines have disappeared (see screenshots in my opening post)

Changing the value back to true instantly brings the problems back. The downside is that my game does not look as good because of the shadow changes, not what I would expect from 4GB and 8GB GPUs.

@Bluebellflora 
iMac (Retina 5K, 27-inch, Late 2015); 3.2 GHz Quad-Core Intel Core i5;AMD Radeon R9 M390 2 GB; Catalina 10.15.2
Settings: windowed, no retina disabled, ultra graphics
I own all packs

I do think I'm seeing lag in my (system) computer in general that I didn't before - even when I hover over icons in my doc (slight hesitation). I have been noticing in-game lag in this time-frame. This lag did not seem to lessen if I changed my graphics from ultra to high.

I found the game plants both from build/by and from debug had the lines in the direct sunlight but not shade. However, if you notice in the screenshots not all items from build/buy are doing this (notice the mailbox), plus the 'live' plants in the garden (my harvestable ones) don't have lines. Neither do the (debug) trees.

Edit:
A few more bits of info...
I play in windowed mode and with retina disabled. 

FPS info:

• With true in RenderShadowsEnabled on medium, high, and very high graphics (per default ), the FPS with a lot in live mode averages in the low to mid 20s. In build mode the average seems 25-30, so a bit higher.
  With retina enabled: Live = av 10ish
                                   Build = av12ish

• With false in RenderShadowsEnabled on medium, high, and very high graphics, the FPS with a lot in live mode averages more mid 30s to low 40s. In build mode the average was a bit higher - mid 40s.
  With retina enabled: Live = av mid-20s
                                   Build = av10ish

As an aside note...
When I tested with retina enabled and the with the settings at true (the default for medium, high and very high), there was a noticeable increase in lag in-game plus it even froze my game to try to exit. I had to force-quit.

My computer specs:

• Macbook Pro, Early 2015
• 2.7 GHz, intel core i5
• 8gb RAM 1867 MHz DDR3
• Intel Iris Graphics 6100 1536 MB

Game Version: 1.60.54.1220

I'm back with the same issue that I thought I'd fixed in my last post, and this time I think it's the game. I have already done a factory reset of the game and Origin. I have tried to repair the game through Origin. I started a new save file on a new game with no mods, no tray, no saves, and not even my Options.ini file added in. The lag is pretty significant even though there's nothing added to the game (graphics on low as well, though i usually play on high or very high with few issues). When I add in just one save file and try to play it (again no mods!) it is much worse. Completely unplayable again- the sims take forever to move and the game takes forever to load. Couldn't find another thread of someone having this exact problem on Mac but if there is one please link me to it. Very desperate here.

The save file is 23.6 MB. No, I haven't put any mods at all in yet.

I tried it with a new save game too though and had the same issues. Like I opened the game before transferring my own save file, created a random new sim and played in a household with only that sim. Still had the same issue.

How do I turn off retina display? I'm not finding any info for that online.

Okay so playing with retina off is helpful... I put my mods back in and my original save file and it's about as laggy as it usually was before (which isn't much, and I don't mind it because I prefer having mods). But is there any insight on why this would have happened? Was the tiny living patch THAT large? Like I said, I usually have a few issues, i.e. sims freezing in place for a few real time seconds before moving to a new task, slightly longer loading screens, camera sticking at times. But this patch was significantly worse... sims were frozen in place for 5+ real time minutes, the sim game clock would just run along and everyone in the household is frozen.. the cursor won't move so I can select things, and I timed my loading screen from opening the game all the way to getting inside the household to play and it was legitimately 20 minutes long- it's usually just a few minutes like less than 5.

I guess my question is- do I always have to play with retina off now? Or is there something else that could be causing this?