I’ve played a lot of hours of demanding games on the Deck; I do all of my gaming on the Deck. I played a lot of Deep Rock Galactic (600h), too much Cyberpunk 2077 (400h), a little Kena: Bridge of Spirits (4h), and a reasonable amount of Horizon Zero Dawn (100h).
These are fairly demanding games and while the default graphics settings grant you around 30 fps which is good enough, I’ve installed cryo utilities and played around with the graphic settings a lot, and generally average closer to 45 fps.
For Cyberpunk 2077, the issue is primarily driving, because moving faster increases the graphical load, and this significantly reduces fps to something like 20 to 30, and the new area (Dog Town) is also a problem, for whatever reason. Most of the time though, I’m playing at 40 fps in non-demanding circumstances.
For Deep Rock Galactic, the issue arises mostly at higher difficulties with more numerous enemies, but with cryo and the right graphics settings I’ve pushed it to a steady 40 fps all around, without making it look like a potato.
Horizon Zero Dawn runs actually totally fine and gets you an easy 50 fps in most circumstances, but riding along a mount and travelling in the world causes weird huge lag spikes (in SteamOS, probably due to Proton related bug), but I find them uncommon enough that I’ve played a good 100 hours just fine.
Kena: Bridge of Spirits on the other hand runs like an absolute charm, hitting a good 50 fps all around without lag spikes in general once you use a little more up scaling than the default.
I don’t know what games you play but I would recommend playing around with the settings, maybe pushing the up scaling a little further, maybe disabling some particles, depending on whether you care more about performance or quality.
Horizon Zero Dawn runs smoothly on Windows from what the internet claims if you install Windows on the Deck it runs just fine, but the internet could be wrong.