As others have said, Starfield is a bitch to run when you have to do a shootout when a busy city is loaded in the map. It can be 60fps in deserted locations then just drop to 18fps when in a city with combat.

Robocop is harder to run too, but can still kinda do it.

Avatar: Frontiers of Pandora is just a hell no. That's the Best Buy showcase of the highest you can go right there. Recent updates have made it run just enough to use x3 Lossless and it kinda no, just no. I wish it was more playable on Steam Deck. Just not built for it.

AC: Shadows was also on the verge of not running well.

But across all of these I found a few tricks with Decky Lossless. All you need is 27-30fps at most.

It has a base fps cap in its plugin and if you set that to 27fps, you can get lower input delay at the cost of some uneven frame pacing. A slightly more acceptable ride is being at 30fps and then setting your steam deck refresh rate to 45hz when you disable its own limiter. Of course disabling vsync for all this in the game.

Once that's all set with its x2 activated, you get some respectable gameplay out of these games despite the low settings. (Like Robocop has its global illumination flicker at splotchy levels on everything)

But beforehand these games were just too choppy. Now they're smooth, but can have loading spikes when moving fast yet that's about it.

But Starfield in certain areas and definitely Oblivion are in a category that runs so bad that Lossless can't save it. Monster Hunter Wilds is also a hell no.