Website performance check
How long a page takes to arrive, and what it costs on the way.
Checks run in parallel, and slow ones time out rather than hold up the rest.
What gets checked
What this tells you
Speed is cumulative. Every redirect hop, every slow first byte and every oversized asset is paid for by the visitor, on their connection and their device rather than yours.
The carbon estimate is the same numbers read a different way: bytes shipped, multiplied by everyone who ever loads the page.