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This extension is so great, but it slows down rendering of html email previews.

Ta opinia jest o wcześniejszej wersji dodatku (5.11). 

I am aware of performance issues (in Thunderbird 102) and am working on it - see issue 283. If you could add some comments there it would be very welcome!:
https://github.com/RealRaven2000/QuickFolders/issues/283