All I can say here is that we are aware of the slow FCP and LCP and have long term plans to improve them.
Specifically @eviltrout is upgrading us to ember CLI. Once that is done we can start thinking about and possibly experimenting with code splitting and other tricks.
There are no easy tricks here, we use a CDN, we are very deliberate about how we load stuff we spent countless hours optimising here but fundamentally we use JavaScript to render our pages and delivering/parsing/running the JavaScript takes time on the first load.