Updating panel elements (e.g. Clock or system monitor plugins) cause lag / stutter in video playback and games
I have the same problem that is described in this old issue in xfce4-systemload-plugin: Whenever something in the panel automatically updates itself, be it the seconds on the clock or one of the system load graphs, I get a small stutter during video playback (YouTube) or games or the unigine-superposition benchmark.
It does not seem to matter if it comes from a built-in feature (displaying seconds on the clock) or a plugin (system monitor). Removing the system monitor plugin from the panel and disabling the seconds on the clock stopped the stuttering. Suspending the processes of the plugins with 'kill -19' also worked, but only on the plugins as the clock doesn't have its own process and I don't want to suspend the whole panel.
This is how it looks like: I let the unigine-superposition benchmark run in game mode (standing still for most constant frame times) and traced the frame times with mangohud. Displaying seconds on the clock has already been disabled but the frametime spikes produced by the clock looked just the same as these:
xfce4-systemload-plugin enabled with update interval 500ms
xfce4-systemload-plugin removed from the panel: