I took a look and a couple of days back there was a spike in network traffic that was mirrored by a corresponding spike in CPU usage, which has remained persistently high since. It's related to one of the Windows system processes and I am investigating further.

EDIT: This spurred me on to carry out a long overdue upgrade, the calendar's new home is a WIndows 2019 EC2 instance and MySQL 8.0. Hopefully performance problems are behind us now.