So the tooltip apparently states:
Reduces autofire spin-up time for compatible weapons.
I have only had one match where I even found this hop-up for the LMG, but still couldn't find a Deovtion LMG anywhere to test it out. It's a Legendary rarity hop-up so it's probably rare enough to not be found commonly.
I'm wondering how effective this attachment actually affects the Devotion LMG. To start with, the gun already has a painfully long spool up time before max fire rate is reached, so I'm wondering if it's even worth getting this attachment when it means sticking with the Devotion long enough to hopefully find it through the course of the match.