I doubt flutter will be significantly leaner for real world, full featured apps, like slack. After all flutter is reusing Chrome's building blocks, with Skia and many other render related libraries, with similar IPC architecture for security, etc. They've replaced the js run-time with dart.
See download sizes here [1], if they remind you electron app sizes [2], it's because flutter is a slimmer (not by much) Chrome.
Flutter will suffer from huge update sizes like electron, but may be faster out of the box, and slimmer (not by much) in RAM. However, using Carlo [3] or Lorca [4] can yield similar benefits, without learning dart, or dealing with brand new flutter edge cases solved years ago elsewhere. In my opinion, flutter is not yet a better mouse trap for desktop apps, and even for mobile, ionic 4 is plenty performant
[1]: https://github.com/flutter/samples/releases/tag/gallery-v2.0
[2]: https://central.github.com/deployments/desktop/desktop/lates...