Hello,
I applied this mod to my 180K SPM base and I see no impact on the UPS.

What did you mean by "huge UPS optimisation"?

Theory: every inserter asks every slot by the chest, every tick. Less slots make better performance.

https://forums.factorio.com/viewtopic.php?f=49&t=76389

https://www.reddit.com/r/factorio/comments/a0gafd/does_locking_slots_in_chests_help_ups/

Thanks for the clarification. That makes a lot of sense.

The base uses mostly loaders. I recently switched the base to mini-loaders mod.

I also read about this to be true with robots.

However I did not see any UPS difference.

How much did it drop in your base?

I have no time for big play, the last time when I completed the game was with 0.15.