From personal experience and by observing VJ + listening to hours of YouTube show on immigration with live callers.
In my case, it was a simple case, but everybody who filed the same week or two were delayed by 4-5 months.
When I filed the average processing time took 7 months. My approval took 20 months. No RFE, no interview. That was for I-751 but you can see similar experiences across the board from AOS and visas to N-400.
Just go through threads on VJ, you'll see
- people getting naturalization certificate in hand while their account shows 7 months until decision
- people waiting many months over the estimated time because it keeps jumping up and down