In general, you can optimize an Oracle JET application the same way that you would optimize performance for any client-side HTML5 application.
There are many online resources that provide tips for performance optimization. For example, the Google Developers website describes processes for:
Minimizing round trip times between the client and the server
Minimizing the payload size (downloads, responses, and cached pages)
Minimizing request overhead (upload size)
Improving browser page layout
Optimizing performance on mobile devices
Most of these recommendations are up to you to implement, but the Oracle JET framework includes features that can reduce the payload size and the number of trips to retrieve the Oracle JET application's CSS.