How are speed modifiers calculated?

There are three options to add additional robots to a slot. How are these 3 modifiers calculated?

Are they added to a total of +17,5% or is there another system? I tried to figure this out myself, but it seems like a really strange system to me.

Any help would be really appreciated.

Each additional robot will reduce the time taken by a slot to reduce the time for production.


The tier 1 Chassis Assemble slot will take 12m58s to complete a single run.
With a second par of robots it comes down to 11m42s.
A third set of robots brings production run time down to 10m57s and finally the fourth set of robots bring the production run time on that slot down to 10m30s

That’s not what I meant. I can read tooltips.

What I meant is how exactly are the modifiers calculated? Because to me this system seems really strange.
I already put some numbers into excel and they don’t make any sense:

So if anyone knows how this is possible please tell me.