Was one of my first questions when i only joined the game. Still has no clear answer, as far as i’m aware. You can browse this thread:
Do note that the chart is incorrect (because passive Ghoul is affected), as well as most of the other answers. It appears that the phrase i used there “-6 Seconds to All Current Cooldowns of Active and Devotion Skills” is as close to the truth as you can get in one sentense. I suggest to test all the devotions yourself individually.