Increased build speed produces sentries more often.
~ BTD6 description
Faster Engineering is the 2nd upgrade of Path 1 for the Engineer Monkey in BTD6. It increases the production speed of sentries from every 10.0s to every 7.0s. This allows up to 4-5 Sentries to be placed on the map at once per Engineer (5-6 Sentries with the "Vigilant Sentries" MK).
It costs $340 on Easy, $400 on Medium, $430 on Hard, and $480 on Impoppable.