Under macOS 10.13.4, the "delay-until-repeat" for the keyboard is about 1.5 seconds, comparable to the "Long" setting on the System Preferences>>Keyboard>Delay Until Repeat slider. Moving the slider to "Short" speeds up the delay for everything else, but XQuartz doesn't seem to getting the message. I don't recall this being an issue with the previous version of macOS (10.12.x)
It looks like changes in the "Delay Until Repeat" slider do not take effect until restarting XQuartz. I noticed that after rebooting the machine (for a different reason), the delay matched that of the rest of the system.
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