mounting iron sights on a TEC-9 raises the maximum aim time, while the same mod mounted on a H&K UMP45 lowers the aim time. Which one is a bug, or is it “working as intended” ? 
Considering improved irons are supposed to increase accuracy and not affect aim time, possibly both. Of course, having handled both platforms, the TEC-9 is not exactly an ergonomic design to sight down. The UMP-45 on the other hand is. It MAY be working as intended.