I was experiencing some very annoying pauses & hangs in Thunderbird in Linux. The application would hang when launching, and periodically while open as well. CPU and RAM usage were not spiking, but the process status would show up in top as D (uninterruptible sleep - usually IO, according to the ps manpage).
I tracked down the issue as being related to the Lightning calendaring extension. Lightning has an option for caching calendar entries for remote calendars. This seems like it would be a feature that improves performance, but as it turned out, it caused a lot of app-stalling IO. Each remote calendar has a Cache checkbox in its settings. Unchecking this immediately put an end to the pausing issues.
What clued me in was this bug report, which is almost 2 years old and still “unconfirmed”.