Firefox pgo use flag, xorg and cpu usage

[Darknight]

I recently discovered the pgo use flag which is supposed to increase firefox performance by increasing build time.
I had to add a couple of use flags to enable pgo. One was xvfb for xorg-server.

First: why those extra use flags? How are they related to a gcc feature used to compile firefox?
Second: I noticed some pretty high cpu usage by X when loading pages in firefox, when I first open the browser one cpu core goes up 100% and stays there until most pages (tabs) are loaded. I don't remember this happening without pgo. Is this related to xvfb? Are there xorg or nvidia-drivers options I can use to help my poor cpu?

[chithanh]

pgo will run Firefox code inside a virtual X server in order to generate profiling data for the compiler.

[Darknight]

Thanks, that explains the use flags.
Could the cpu usage be related to a more efficient firefox code generating more frequent calls to X functions per time unit?