On 12 Dec 2005, Ingo Molnar announced authoritatively:This value is 0 on my debian system - as recommended for the kernels
does this mean X defaults to nice level 0, and then if you renice
Firefox and X by +10, everything is fine? Or is Linus' suspicion, and X
defaults to something like nice -5? (e.g. on Debian type of systems)
Your latter suspicion is correct, on Debian at least: see the setting of
nice_value in /etc/X11/Xwrapper.config.