Update (March 2026):
The behaviour described in the OP seems to have changed. I’m no longer seeing the rate-limited loading behaviour in the FullCalendar view.
However, I am now experiencing rate limiting when accessing the new /discourse-post-event/events.ics export endpoint instead.
This appears to affect external calendar subscriptions rather than the internal FullCalendar UI.
I was able to successfully subscribe to the calendar in Moodle by:
1. Changing the URL scheme from https:// to webcal://
2. Adding &after=DD-MM-YYYY to the query string
This effectively reduces the number of exported events and prevents the rate limiting.
This confirms the endpoint supports date-windowed exports, and using ?after=DATE for subscriptions is an effective way to keep the ICS feed lightweight and scalable, even if the forum has many historical events. Combining webcal:// with ?after=DATE gives an ideal setup for external calendar clients: only future events, small ICS file, and stable performance.