> your concern specifically applies to images that your application server is rendering custom for each visitor?

It's more if you accept image uploads from users in any way. I experienced a ~10-20x increase in processing time when accepting AVIF images, and I found that I couldn't rely on sync functions for that. I had to both change to async and upgrade hardware in order to support it at scale. I run a small reddit-like site that accepts media uploads and the end result was I had to choose between unshipping AVIF or doubling my server costs. I found that the image savings over webp weren't worth it at the time.

The same story is likely true for JXL until there's hardware encoding.