I'm not OP but I've worked on JUCE plugins with React UIs for JUCE8 web view.

The UI load is pretty instantaneous.

Everything uses the native web view. This means that on macOS you get WebKit and on Windows you get Chrome (or sometimes IE).

So it's generally faster than Electron. In the project I worked on it was not noticeably slower than native UI.

Memory usage is really close to what you might expect from any other app running native web view.