Diagnosing large delay in file sync to Docker for Mac
I figured this out, and it was related to the opcache settings for PHP in my docker image. The opcache re-validate frequency was set to 60 seconds, meaning that PHP caches files in memory for that long before referring to the filesystem again. Changing this to 2 seconds resolve the issue, but adds a significant performance penalty :(.

See more info: https://drupal.stackexchange.com/questions/253111/delay-in-code-changes-on-custom-pages-in-drupal-8-in-a-docker-environment