Forty frames per second does not sound dramatically faster than 30. It is only ten extra frames each second. On Steam Deck, though, 40 FPS became a sweet spot because frame time—not the raw frame-count difference—is what your hands and eyes actually experience.
At 30 FPS, each frame lasts about 33.3 milliseconds. At 60 FPS, it lasts about 16.7 ms. A 40 FPS frame takes 25 ms, exactly halfway between those two frame times. That is why the jump from 30 to 40 can feel much bigger than the numbers suggest.
Frame rate and frame time are inverse measurements. Divide 1,000 milliseconds by the frame rate and you get the time budget for one frame: roughly 33.3 ms at 30 FPS, 25 ms at 40 FPS and 16.7 ms at 60 FPS.
So moving from 30 to 40 FPS removes about 8.3 ms from every frame. Moving from 40 to 60 removes another 8.3 ms. In responsiveness terms, 40 FPS sits at the midpoint even though the headline frame-rate number looks much closer to 30 than 60.
That does not magically make 40 FPS "as good as" 60. Sixty still updates the screen more often. But it explains why a stable 40 can feel like a surprisingly large step up from a stable 30.
Valve built the Deck around that compromise
Valve added adjustable in-game refresh rates to SteamOS 3.2, letting the original Deck screen run from 40 to 60 Hz. Valve explicitly described 40 Hz as a sweet spot for responsiveness, consistency and smoothness, while letting players save the setting per game.
Matching 40 FPS to 40 Hz also gives each rendered frame one display refresh, which produces a regular cadence when the game can actually maintain the target. PC Gamer testing and early technical coverage likewise highlighted 40 as a useful balance point for demanding handheld games.
The caveat is stability. A game that constantly drops below 40 can feel worse than a properly paced 30 FPS target. Battery results also depend on the game, power limit, graphics settings and hardware load; 40 is not an automatic battery-life cheat code.
Why the setting stuck
Steam Deck made 40 FPS popular because it gave handheld players a third option between "cinematic 30" and "aim for 60 at any cost." The number happens to line up with a neat piece of frame-time maths, but the real win is practical: smoother response than 30 without demanding the full performance budget of 60.
That is the tiny technical fact behind one of the Deck's most useful tuning habits. Ten extra frames per second; 8.3 fewer milliseconds per frame; a much more noticeable change than the first number implies.
The practical catch is consistency. A locked 40 FPS target only feels good when the game can actually stay near it; repeated drops below the refresh target can undo the smooth pacing advantage. That is why the setting works best as a per-game choice rather than a universal Steam Deck rule.
Reporting notes