In the docs about subscribing to external calendars, you state that you sync third-party calendars like Google, Outlook, or Nextcloud only every 8 to 16 hours (https://proton.me/support/subscribe-to-external-calendar). This is by far too slow to make third-party subscriptions useful. If my iOS calendar app can sync a Google calendar within seconds, you should too. Please reduce the sync time to ideally less than 1 minute, but 5 minutes at most.