Documentation of this seems to have disappeared into the ether, but I'm reminded of the time that glxgears (or a distro package of it?) briefly turned off FPS output by default, with the argument to turn it on being something like --i-acknowledge-that-this-tool-is-not-a-benchmark

Basically, people were complaining about "major regressions" in glxgears frame rates that were really just minor fluctuations in how long it took to clear and swap buffers, since glxgears does almost no actual rendering (by modern standards).