I was looking at bug 424715 profiles, and we spend a fair amount of time reflowing every single placeholder on the single existing line in the testcase. This gives O(N^2) behavior, since the testcase repeatedly appends placeholders to the line. The main time-sinks are vertical alignment of the line and the actual reflow state construction for the placeholders. I'm not sure we can do much about the former, but the latter should be pretty easy to fix if we create a separate path for placeholder reflow in nsLineLayout::ReflowFrame: placeholder reflow is dead simple and doesn't use the reflow state in any way. Is it worth doing something like that?

I'm not sure.