Hi, I am using Thunderbird for years and this issue started to happen last week. I am using a Teamup calendar as an external (readonly) calendar, this is the URI: https://ics.teamup.com/feed/ksrcmy4ajs6stupbg6/5826514.ics Apparently, from last week when it is enabled, Thunderbird needs about an hour to start and memory rises to 3 GB. I tried older TB versions while I was investigating what is the issue with the strange behavior, from TB ver. 102 and later, but it is the same. At the end I found what was causing this memory consumption and startup slowness and I disabled the calendar and TB works well. If you follow the link and download ics file it seams ok, it imports to other calendars lika Outlook, Calendar etc. easily. Please investigate and update and I will send any additional info if needed. Best regards, Vladimir