LISTEN/NOTIFY has known performance issues that aren't documented but regularly surprise users in production – my customers and I encountered some of them multiple times.
It's probably a good time to consider improving this area, but while it's not happening, I propose documenting the risks to help users avoid incidents (backpatching to all supported versions).
The proposed docs patch to the LISTEN/NOTIFY docs includes words about: 1. Global lock during commit affecting all databases in cluster 2. O(N²) duplicate checking performance 3. How to diagnose (log_lock_waits example) 4. Alternatives (logical decoding)