They're used in this article on resource hints: https://medium.com/reloading/a-link-rel-preload-analysis-from-the-chrome-data-saver-team-5edf54b08715
I can't find then used anywhere else though, nor any definition of their meaning.
My colleague found the answers as soon as I finished posting:
RTT := Round Trip Time
TTFCP := Time To First Content Paint
HRBR := Hidden Render Blocking Resource