User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:72.0) Gecko/20100101 Firefox/72.0
Steps to reproduce:
Where a filter has a large number of entries (~300), it takes a significant time (~20 seconds), to load the filter into the window for editing. After cancelling and returning to the Messages Filter page, it then takes a significant time (~20 seconds) to mouse click onto an alternative entry. Sometimes the dreaded spinning coloured ball appears. Eventually it appears to resolve itself and it is then possible to select any entries without delay, either with the mouse, or the up-down arrow keys.
This problem appeared with one of the latest updates. However, searches show that it had been complained of in the past, in 2011, 2014, and 2017.
The filters run normally on a message folder at high speed. It is only when trying to edit a filter that there is a problem.
I resolved the problem back to a manageable situation by splitting the 300 items large entry into 20 entries of about 15 items in each. Each then loaded into the editing window in a reasonable time (less than 1 second).
To effect this workaround, I edited msgFilterRules.dat as a text file and copied it back. It would have taken excessive time and effort via the editing panel.
Included in the description.
A workaround was possible, as in the description, but this would only be possible by someone tech-savvy and familiar with the internal data structure of Thunderbird.