My immediate example for this is 'gamescope'. It's like a mini compositor that really improves the edges between X/Wayland
For those using Linux outside of a Deck, look into setting it up!
This was my first sentiment as well, but found myself in the same exact position as you. I almost was a little disappointed because I was looking forward to playing with the underlying OS, but even some of these heavy AAA titles run really smooth considering the hardware they employed for the deck.
Oddly enough that is partially to the credit and fault of Sony/Microsoft. games have been stuck targeting 2013 hardware for a very long time, longer than any other generation prior. But it was also needed in a way; the cost/speed ratio of storage was woefully behind the power advancements, and we see that with the crazy load times in the mid-late generation games of gen 8.
On the same end, mobile has been advancing in huge strides and there were more x86 hardware that could run on lower wattage. Valve simply saw a hole that only certain Chinese manufacturers were trying to fulfill, and they used their market advantadge to slash prices. Honestly surprised Microsoft didn't try this first, given their dabbling with Surface (I know Sony has long since been burned out on handheld gaming)
As a dad with two very young kids I don't get much time for gaming, but the Deck helps me find that time and I am loving playing through some of my backlog of controller-friendly games.
Also a plug for https://www.protondb.com/ which has thousands of user-submitted reviews and optimization fixes/tweaks for playing games on the Steam Deck.