Thunderbird "not responding" when deleting reminders

I have used Thunderbird and its diary for years, and make extensive use of reminders in the diary (my memory is not what it once was!). Latterly, when deleting or postponing a reminder, Thunderbird goes into "not responding" mode for many seconds or even some minutes. I have Thunderbird linked to both my gmail and my protonmail email accounts. If I look at Task Manager when Thunderbird goes unresponsive it seems to be the connection with protonmail that is the problem - see the attached screenshot. Help, please Andrew