My Husband read that too! He found it on the uk gov website from the confirmation email. "80% of the priority settlement visas are processed with in 15 business days" So that could be the same thing? My husband and my family keeps saying, you could get it faster! I would be the same way and 30 days.. thinking okay I'd really like to see my man now!! So here's to staying busy and staying positive! 
I believe that's from the May processing times published on the UKVI website, which say:
- 2% decided in 3 working days
- 60% decided in 5 working days
- 72% decided in 10 working days
- 78% decided in 15 working days
- 85% decided in 30 working days
- 98% decided in 60 working days
- 100% decided in 90 working days
However, those times (2, 3 5, 10 and 15 days) seem very suspect, since almost everyone on this forum who has applied priority in the last 6 months has experienced processing times of at least 30 working days and I don't think I've seen anyone get their settlement visa in 15 working days or less since January 2018 (and that was only a couple of people). Before that they hadn't been processed that quickly since February 2017.