The UI stutters *only* when I press the Recent Apps button to switch to a different app. I had already wiped the cache parition.
How do I find Good Lock? What about Theme Park?
Good Lock is not available for my region so I installed NiceLock from XDA which is the same thing except it's not region-locked.
It's good! I can temporarily try a different, much smoother task manager UI while waiting for Samsung to fix the Recent Apps laggy transition bug.
[Bug fix/workaround for laggy Recent Apps screen?]
I've been playing around with NiceLock for a while and managed to make a nice light Holo Blue theme for myself using Theme Park. I also installed the original Material Design icon pack from Galaxy Themes to get much better icons and to get rid of, for the most part, the Samsung-style icon borders which I personally find annoying. So I'm quite happy with the look on my S9 now.
However, all the Recent Apps styles offered by the Task Changer module are quite frankly, ugly versions of the original Android Recent Apps styles, with even more cartoonishly rounded corners and unnecessary animations. The animations though annoying are all very smooth. The Carousel style is the closest one to the default Android Pie/10 so I used it for a few days. I found it hard to swipe between recent app pages. The only good thing is that there is a "Center the currently running app" option (the default Android Pie/10 behaviour).
I then disabled it and went into Developer Options in Settings on the S9, and found out that the settings below fixed the sluggish Recent Apps bug and enable the "Center the currently running app" behaviour!
Window animation scale - No need to change, but I set it to .5x
Transition animation scale - Off
Animator duration scale - .5x
The only problem I have with this workaround is that sometimes the Close All button becomes invisible yet functional.