Here is a Java/C++ audio workstation software,

https://www.bitwig.com

And here is a must read book to do great UIs in Swing,

http://filthyrichclients.org

Here are examples of Java applications based on Microsoft's Fluent UI design,

https://www.pixelduke.com/java-javafx-theme-jmetro/

Performance is fine on all of these compared to Electron. Every non-native toolkit needs a lot of work to look good. But it's usually less work than native on every target.

Swing is hardly slower than Electron.

And what's unacceptable about wxWidgets performance?