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When put into Hold mode, X11 handles the power switch as if it's being pressed, and it generates lots of keypress events, passing them to the foreground application. Since /dev/input/eventX does not seem to continuously trigger, I'm guessing it's due to key repeat.

To reproduce:

• Install xev and open it in the terminal
• Put the power key in HOLD mode
• Notice the "time" value changing rapidly

In addition to causing more work to X, it also floods the active application with key presses, resulting in increased CPU usage. In my test, switching to HOLD while Firefox is in the foreground causes it to use 6% CPU and X an additional 2.5%.

In addition, the key-press events continue even after the switch is taken off HOLD until another key is pressed.

A possible solution to this is to blacklist the power-button event device, to prevent evdev from listening to it (the key is handled by pndevmapperd).