Currently, the only time it is possible to see the efficiency percentage of a building is when the building is actively producing. If it’s waiting for resources, you can’t see the percentage. If it’s full, you can’t see the percentage.
This causes issues because, for example, if you don’t remember which buildings have been boosted by the spaceship’s communication array and computer deck, you need to actually build them and wait until they start producing to see. It also makes it very inconvenient for players to plan their building layouts in advance, because not being able to see the efficiency makes it harder to calculate how many of each building will be necessary.
I would recommend either putting that info someplace else in the building’s info window (the one with the person on top of it) or just displaying it permanently in the progress bar.
To be fair, there is reasoning for not doing this: visual clutter. The info window already has a lot of numbers and information on it, and adding more might make it look overwhelming, especially to beginners.
So what I would recommend is that when a building is at 100%, instead of displaying the number, just show a little gray circle. Such an unassuming symbol would make beginners not even notice it, which would prevent them from worrying about it when they don’t need to. Then, when they build their first concrete factory right next to a sand/sulfur mine, they will discover the efficiency system and be encouraged to take advantage of it.
When the efficiency is not 100%, the circle could be replaced by an arrow (upwards for >100%, downwards for <100%) and the actual percentage would then be displayed next to this arrow.