How to improve performance of Flutter websites?

I've developed a Flutter Webapp, it's a small and functioning multi-page app with image assets, database connections, and several other features. It shouldn't be heavy but the loading time is very slow and the performance in general is not good.

I'm fairly new to Flutter but I can't understand why its so slow as I've kept it quite lean during development.

Please run my site https://www.coursekiosk.com on https://web.dev/measure/ The main thing seems to be the JS scripts (which is Firebase code for connecting to database etc), and the content painting - which I'm not sure how to make fast without losing the quality of assets used.

I have a feeling it's my overall design/structure which may not be very efficient, and I'd like to know how to develop a website, whether Flutter or not, with performance in mind.

I'm happy to give more information if needed. Thanks for the help.