I thought it would make sense to provide a explanation on why exactly the Deckverse's frame rate target is called the Golden 40 ... and not, lets say, 45 or 50.

The Basics

For this, I first have to briefly go into the basics, even though most of you probably already know that:
Different screens, including the Steam Decks display, offer different refresh rates. The most common ones in the gaming scene right now would utilize 60 and 144Hz.
In other words, the display can refresh the image 60 or 144 times per second, providing new information for your eyes.
Up to now, most commercial TVs usually worked at 60 Hz, which we all got used to. They can render new information up to 60 times per second.
In mathematical terms, this translates into a new image about every 16.6ms.

The 30´s

However, games have become more and more demanding in terms of graphics. But consoles, for instance, might not always be able to deliver 60 frames per second. That's why they have decided to then stick to the lower frame rate of 30 FPS: Yes, those won't supply every new refresh of the TV with new content - but they at least cover every second frame. This consistency is more comfortable for the viewer, although at 30 FPS only every 33.3ms a new image is rendered. And consistency is key - especially when player input is involved. Theoretically it still takes 33.3ms from the moment you press a key until the result of your action is visible on the screen, but since this value won't randomly change, the output won't feel choppy. That's what we call Framepacing.

The 60´s

At 60 FPS, for instance, this would only be 16.6ms, which results in a much more responsive gameplay. This is also the reason why 60 FPS is considered the perfect frame rate by many. At least up to now and outside of the PC gaming bubble. Since not all games can reach 60 FPS on every platform and, most importantly, hold it consistently, you are usually forced to make compromises: Better graphics quality - for instance like denser foliage, higher view distance or simply a cleaner image - or a smoother gaming experience with a high frame rate but less fancy visual appeal?

While current consoles increasingly offer a choice between image quality and high frame rate, in the case of the Steam Deck there is another point to keep in mind! Of course, you can opt for either the lowest quality settings to achieve the most responsive gameplay, or the highest possible quality settings at an acceptable frame rate and go for it. However, since the Steam Deck is basically a handheld gaming PC, we have to keep the limited battery life in mind.
What's the point of having a state of the art gaming experience waiting for you in your backpack if the device only lasts 30 minutes?

The golden 40!

And that's where the Golden 40 comes into play: While you might assume otherwise, a frame rate of 40 FPS offers a frame time of 25ms. That's exactly halfway in between the frame times of 30 and 60 FPS. So even though the Steam Deck only has to provide additional 10 FPS, the real experience is closer to 60 FPS. This also means that the hardware consumes less power: In some rare cases, 40 FPS actually only consumes one watts more than 30 FPS. 60FPS, on the other hand, which usually can't even be kept stable, will bring the Deck to its 25 watts limit in most situations. This can be pushed even further with additional tweaks like limiting the TDP through the quick menu to gain more precious battery life.

On top of that, the Deck's refresh rate can be lowered from 60 to 40 Hz in single steps, unlike on many other devices. In other words, in case of 40 FPS as the target frame rate, we even have a panel that can display new information for our eyes with every new refresh. Which again feels much smoother and more responsive. If you then utilize a game's Vsync option, you can flatten the framepacing even further since it then caps the FPS organically to the panels refresh rate. There's also the option to resort to an inengine framerate limiter some games offer or even limit the FPS through the Deck's quick menu, But which of these three options is the actually the best differs from game to game, since this will introduce inputlag in some cases. An important thing I will try to address in all of my publications about the Steam Deck.

To wrap it up: 40 FPS at 40 Hz has at least three core advantages.

1. [Less power consumption] A 40 FPS limit for instance results in 30 minutes more playtime off of a nearly fully charged battery in the 2018's action-adventure Below.
   
   
2. [Achievable Framerate] It's overall easier to maintain 40 FPS in some games and therefore you benefit from better framepacing thanks to a rock solid frame time of 25ms.
   
   
3. [Better graphics quality] Even if the Deck could provide solid 60 frames per second, limiting the frame rate to 40 frees up more ressources, which can for example be used to increase the visual quality.

There you have it. I hope I was able give you some answers!

Until next time! - Cheers!