Rated 2 out of 5 stars

Lightning is an important way to view your calendars, but slows down Thunderbird a lot. Maybe by a factor of 100x. In order to get a better rating, priority #1 should be ensuring it does not slow down Thunderbird in its entirety. Thanks to all the devs who spend their time making the world a better place with Lightning. Hope you can make it usable.

This review is for a previous version of the add-on (6.2.5).