I tend to play stuff at 40fps because my experience of it is that I feel like I’m getting 90% of the way to 60 (playing a lot of 30 until mid-20s will do that to you!), and then shift up to 60 if I have headroom or the game in particular feels rough at 40. My main machine has a 120Hz monitor and it’s lovely for some games, but it’s something I ‘feel’ on the mouse more than I ‘see’. I just can’t imagine using it on my deck for the vanishingly few games that can hit 90 on the Deck and then caring.
But, I’ve seen claims like ‘setting it to 40Hz will make it run 40 in an 80Hz container’ (some Nerd Nest video or the guy on it who does a lot of battery testing), or the PCGamer review which states the OLED can take “a game locked at 30 fps and triple each frame so it runs like butter at 90Hz”.
Is this just referring to, say, response time / latency?
No you are wrong. The VRR is different because it does it on the fly so you do not need to lock the Hz. Steam Deck does not have VRR, so it has to lock, but can lock to any Hz you want. Is not limited to 90Hz.
Lets say you have a game that fluctuates from 30fps to 50fps. On the Deck your best option to to lock at 30Hz (or an integer multiple). This looses the extra fps but this is not that bad, you get it on battery life. On ROG Ally that has VRR you can live it uncapped and get all the extra fps which is nice (and also have less than an hour battery life)