Are there good articles on the benefits of async over something like PHP-FPM?

At $WORK, we run a lot of 1000s of php servers with 10-20 processes each using nginx. External calls we don’t need the result of immediately get kicked out to an external queue, but we haven’t seen any issues with DB calls blocking the whole server.

Long or slow DB connections are an issue for individual request time and DB load though (the biggest DBs we run handle 1000-2000 concurrent connections).