Rows [0], a spreadsheet app, has its Windows and macOS versions built in Flutter. It's quite fast from my usage. The marketing video they did for Flutter [1] goes into a little detail as to how they did it but they also have a tech talk that goes into more detail [2]. Their spreadsheet engine is open source as well [3].

[0] https://rows.com/download

[1] https://www.youtube.com/watch?v=OEdQXBUPYOE

[2] https://www.youtube.com/watch?v=fv_mzbEtUmU

[3] https://github.com/rows/swayze

Same question, do they have a reasonable component library on windows?

There's https://pub.dev/packages/fluent_ui

and rendering benchmarks