STR: 1. Load this file: https://taskcluster-artifacts.net/XZmhivqwQUKrk15pEu7R9Q/0/public/logs/live_backing.log (Or if that link dies, download and gunzip attachment 9018049 [details] which is gzipp'ed text version of that ^^ file, and open the resulting text file in Firefox.) ACTUAL RESULTS: The first paint takes quite a while -- there's a ~5 second or longer delay before we paint anything. EXPECTED RESULTS: Something should be painted sooner. Note for comparison: Chrome seems to take as long or longer as we do, to *finish loading the file* (same goes for gedit, the Ubuntu text editor). However, we take much longer to get to our first nonblank paint. Spinning this off from bug 515447. Calling this [qf:p3] per QF triage today.

For the record, here's a profile of me loading that taskcluster URL: https://perfht.ml/2AoL2gt

Unsurprisingly, all the non-poll() time in the profile is under one of gfxFontGroup::MakeTextRun, BuildTextRunsScanner::SetupBreakSinksForTextRun, gfxTextRun::BreakAndMeasureText. It's worth seeing how much data the parser dumps into the DOM before it yields to layout for the first time.