Cloudflare APO slows down bbPress submits


Our website runs fine with APO, apart from submitting threads and replies in bbPress. That takes 8 seconds if APO is enabled in the Cloudflare plugin. With APO disabled, it takes 3 seconds. With the entire Cloudflare plugin deactivated it also takes 3 seconds.

What is causing this? Is there anything I can do to exclude APO from submits?