Why do you compare it to discourse? Compare it to stackoverflow or wordpress. In the real SEO world we compete against the fastest, not the slowest.
said it before: "Discourse seems to have similar Google PageSpeed results and is slower on Pingdom Tools and WebPageTest.."
Quote: this site isnt really doing that much wrong.
I know and I really like nodebb, but why not doing everything right? It is not really much to change to make google happy.
Every single minute of your time you invest in your community and content is a lost time, if your site is not as fast and SEO friendly as your hardest competitors.