Nature of the beast; we are a JavaScript application, so the app has to be delivered to the browser.
It is a bit like if you were measuring smartphone app startup times, including the time spent to download and install the app from the app store as part of the “startup” time.
What isn’t shown is that subsequent page loads are blindingly fast in Discourse because the app is loaded in the browser, and we only send API calls and json (the data necessary to render the page) down the wire.