The issue is that thunderbird is apparently built on top of firefox code, so I expect there's a similar level of layers upon layers of bloat as with many electron apps. But this rebuilding project seems to be focused on the UI side of things and changing how things look, so I don't know if it'll actually improve in terms of performance (it might help a bit though, since they intend to remove stuff and clean up the code on the Thunderbird side of things).

Personally, I feel it would be more interesting to turn it into a Firefox extension (and extend the extensions API where necessary), so the resources that are shared could be actually shared. That, or fully embrace K-9 Mail (the android app that they partnered with and which will become Thunderbird mobile) and adapt it for the desktop.