Even though the rest of the properties are based on fast inserters, the latter rotation_speed causes the smart inserters to work slower when compared to fast inserters. There is another double key entry, but as the values are same, it causes no difference.
Wow, good catch, thanks for the report, this one is there for a very long time already.
I asked in the office, and some people actually told me that they knew that smart inserter is slower, and they thought it is by design.
No, it wasn't by design
Haha, that's awesome. I've always known Smarts were slightly slower. It's why on things like the old "passive logistical loop" we had to use 1 more smart inserter than fast inserter, to catch that stray item off a belt that the smart wasn't quite fast enough to get. I think myself and the various streamers I've watched play a lot all thought it was by design!