It is painting 4 i/s, just because of how the reader measures items it wont display that

Neighbouring parallel double painters often stuck each other, cause a bottleneck of whole assembly line. Maybe even a single double painter can stuck itself?

Or maybe it is just because of the reader didnt correctly work dealing with uneven input. You can try another way, using the hub and the statistical information to measure the speed.

Maybe the reader is inaccurate?

I got it. The reader at the bottom reads 3.6 items/s. Maybe that's why.