Out of curiosity, what makes people prefer desktop apps for mail over just visiting the browser version? The idea of downloading a desktop app for something I can do in the browser makes no sense to me. At least on mobile, the ergonomics of apps tend to make the experience better than the mobile-web version.
But desktop software SHOULD be more performant.
Regardless of all the work done in browsers, native UI's still feel better in terms of latency and performance.