Hi Luke,
Thank you for your patience and efforts on this issue, as you know this is an uncommon performance issue: no error messages, occurs randomly, impacts OWA but Outlook client works well.
In this situation, I'd like to suggest you create an entire new shared calendar with the same settings as this one which has issues, assign the permissions to users and check if the issue persists. This step will help us confirm whether it involves the specific shared calendar.
Best Regards,
Bourne Zhang