I thought it was a reference to “Time to Interactive”. https://web.dev/interactive/

0-100, as a concept, maps closer to your idea. I knew about TTL, but didn't make the connection until you pointed it out.

For a non-HTTP specific phrase representing the same concept, I've tended toward "Time to first dopamine hit". :)

That feeling of not being sure if a tool/API/service/SDK/library/hardware is going to work for your purposes and then you get that first example/test/demo running and get your first response...

"Ok, yep, this is good! Now is it gonna let me change this small thing so I can..."

And the positive feedback loop has begun!

It's definitely a metric that impacts developer adoption & is IMO something that needs to be routinely tracked in order to reduce the time taken to get started & catch any unexpected regressions.