nsScanner::ReadUntil() is showing up on profiles for long pages (cf. bug 56854, where it accounts for about 4% of the overall load time). Specifically, it looks to have an inordinately high fan-out into the string routines; see bug 56854 for details. Filing this as a task to examine its implementation and improve it.
Already have major improvements here in bug 65431.
Excellent. I shall eagerly await the resolution of that bug, and re-evaluate!
This is no longer a hotspot. *** This bug has been marked as a duplicate of 65431 ***
Marking it Duplicate as per above developer comments.