When I went to bed last night with Final Fantasy XVI installed but unplayed on my Steam Deck, I saw that Valve had placed the mood RPG on the portable device in Unsupported status. Blame it: “not working well” on the internals of the Steam Deck despite tweaking the settings. Damn it, Clive.
I tried it myself this morning, and while it could be argued that the literal interpretation of “unsupported” doesn't apply to FFXVI – the Steam Deck can still boot it up and run it without it exploding in someone's hands – it does ask a bit too much of the hardware. The low quality preset and FSR 3 on Ultra Performance are enough to keep me above 30fps in many fights, but other areas easily drop that into the 20-25fps range, or even the 10s. That's not particularly pleasant to play, even if you're still technically playing. FSR 3 can provide framerate boosting as well as framerate reduction, but it only manages to make things worse by causing heavy stuttering in exchange for a handful of extra frames produced.
(Strangely, nothing performs worse than cutscenes; I've seen them drop to around 17-20fps for no apparent reason on several occasions. But at least that pre-empts any potential annoyance at being capped at 30fps on PC.)
Also, this isn't part of Valve's Steam Deck validation criteria, but I can and will complain about it anyway: It's a 152GB installation. 152GB! So even if you decide to be immune to framerate drops, FFXVI will eat up the lion's share of any Steam Deck SSD or microSD card. 152GB, to be honest. You could fit twelve copies of Dark Souls II in there. Or six copies of Metal Gear Rising.
Meanwhile, after Warhammer 40,000: Space Marine 2, that means you're playing two big-budget action games in a row, with Grumpy Man With a Sword lagging behind Steam Deck performance by a fair bit. As for how the new PC port of Final Fantasy XVI stacks up on more traditional PC hardware, I'm currently locked in the benchmark dungeon trying to find out, but I don't think you'll want to take your chances with anything below the minimum requirements of an RX 5700 or GTX 1070.