After struggling several weeks on my megabase because low ups, even if it was a kind of starter megabase, I manage to found the culprit : this mod, and an other one with even bigger chest. Basically, factorio is not optimized for big chest. Every time an inserter try to pull or take inside a chest, it scans all the slot used in the chest. So if you want to save you a big headache later, remember to limit your chest (or your station). I just go back from 20 UPS to 60 UPS thanks to that.

Hey Teneombre (or anyone else with insight here). When you tried to save UPS did you limit the number of stacks within your chests could take, or did you set some conditions for your inserters to 'limit' your chests that way? I'm not 100% sure which you did.
I'm starting a megabase myself and was worried of adding any mods to it.
Cheers.

I limited the number of stack of each chest. Take especially attention to the steel chest since the stack number can be really huge. If you have time you can also install the quantum warhouse - stack fix (or something like that) mod and manually add the merged chest into the option (work only on the newly build chest, take care).
Bassically, the way I handle that was : stack number limit on imput station (usually a train so around 160 stack. still a lot but way less than 6130 (or something) and disabling output station over a number (usually what was needed for one or two minutes). If the train deliver more, it can still unload and go back to the imput since there is not stack number limit for the output station.

Hello, Teneombre
In other words, the information you provided is
Does using a long chest like 1x55 or 55x1 have no effect on the UPS drop if I only use a limited number of stacks for each chest (less than 500 stacks for example)?
Of course I would like to have a larger capacity of the chest, but what is more important to me is the length of the chest, such as 1x55 55x1, rather than the capacity of the chest.
So a small UPS drop is inevitable, but unless there is a big UPS drop, I try to use a chest like 1x55 or 55x1.
It's a very important issue and I'd like to get some help.
Thank you.

That's it. The number of item stacks inside the chest matter, not the size of the chest itself. For exemple, in my current megabase, I limit my chest to 160 stacks (a 1-4-1 train). It's still worst than a normal set up where each inserter scan for a maximum of 48 stacks (a steel chest) but you avoid balancer and stuff like that so it's a fine trade off for me. I think I will try to look at quantum storage and see if I can use the code to modify modular chest.
I gave a try to quantum warehouse fix mod but unfortunately, seems to not work, even if I added the chest from modular chest in the option.
So yeah, the only good option is to limit the number of stack of the chest.
Fun fact: if you use mini loader, you can use 15 bluebelts and limit your chest to only one slot and still be able to load a 4 wagons train with 6 stack inserter for each wagon.

Let me summarize and ask if I understood what you said well.
If I use a 55x1 size (of 6144 stack) modular steel chest, will the UPS drop less if I use a limit that allows only a few stacks (ex.48 stack)? And even if I don't use the stack limit, is the condition the same if I only have a stack with a few items in the box?

So, for example:
00. A steel chest full of 48 stacks
01. 48 stack limit & 48 stack filled (when using 55x1 chest)
02. Unlimited & 48 stack filled (when using 55x1 chest)

Are the above conditions a similar UPS situation?

According to my knowledge, yes.

Thank you :)