Most users don't change
any settings. Which is why you want sane defaults. I first realised this (very much to my chagrin) when screen-resolution settings were first reported via JS. It was obvious (from the low and poor values selected) that the vast majority of users, under Windows, never right-clicked their desktop and tweaked display settings (800x600 @60Hz on CRTs, or
worse, being the most common values).
The only users who (in significant number) change settings are power users. A small minority in any regard, but also the ones who then are more likely to defeat various tracking and feedback systems -- disabling feedback or preemptively blocking sites and systems which profile system performance, including Google Analytics, New Relic, etc.
Yes, if you can instrument within your tool (Web page, app) for response, great, but that's More Work For You, so it's far too easy to fall back on standard services. Which your best users are most likely to block....