It's crazy that it's slower than an Electron app! Like most HNers, I fully believe that any JavaScript is slower than native, so how can this be false for this app???
The post said that Thunderbird is an app running on top of Firefox, so it's not much different than an Electron app. There are probably many more layers than a native one. Slowness is too new expected but actually I never felt that Thunderbird is slow. I use it daily.