Little misinformation in this thread - and as an owner of the 2018 (A12X/Not A12Z in 2020 model, a slight clock bump) - mine is fully loaded of the '18, w/1TB (which also gave me the 6GB RAM). I also have both an M1/512GB/8GB RAM and an M2/2TB/16GB RAM as we use them daily at my business.
The 2018 - other than the DaVinci Studio limitations, (1080p v 4k) I've YET to see ANY type of slowdown. Glitch. Frame drop... or anything of the sort.
I play Diablo, Flight Sims (RFS, Aerofly 23, X-Plane and Infinite Flight, perennial subscriber to each), other RPGs, COD, and other frame rate/fluency dependent games, my favorite of all being Real Racing 3.
That said, the 2018/2020 are a couple of the best deals on the preowned or even the refurbished dept with Apple. They're genuine bargains and 4GB of RAM is NOT a limitation today. Will it be tomorrow? Yeah - when 8GB is standard and the developers make more money developing for the masses (think iPhone 7/8/X right now or iPad original or second round of Pros before the 12.9" - when the iPad Pro was a 9.7" then the 10.2"
I love my loaded M2 but its for a completely different project the muscle and the memory and the storage and the cellular capabilities are necessary - even though its an option anytime, I always reach for the 2018/2021 models at home. They're as capable and much more tailored to my use, my apps, prefs et al...
But when it comes to watching the movies on the iPad there's no displays available that compete with the latest 12.9" Pro models - HDR etc
ENJOY!!! It's ONLY a Great Choice!
J