Mine was also around the 15 months duration, Kanton Luzern, but facilitated naturalization. But I didn’t really speed it myself. After I got the invitation letter to apply, I took 3 just months to submit the documents to SEM, delayed my interview for 1 to 1 & 1/2 months due to personal reasons, and in the last bit I didn’t push the Gemeinde consistently for the documents and entries I needed in the system for the Passburo appointment which added a few weeks. So in some cases timelines could be reduced to even 12 months for clearcut cases.
Seems like they have increased their processing speed or resource capacity.
For ordinary naturalization, 15 months seems already priority processing, but clearcut (simpler) cases are generally faster, as it depends upon case complexity.
If 15 months is becoming the norm, then people should be generally happy about this.