From time to time it happend that my keyboard behaved in a funny way, although I were not aware that I pressed any specific keys.
In the past I resolved the issue with a restart of the X server but as it started to bug me I had a closer look and the solution was actually quite easy.
A look into the /var/log/Xorg.0.log showed the following line:
(II) XKB SlowKeys are now enabled. Hold shift to disable.
A quick search on the net showed a good few bug reports. Though it’s a feature and not a bug.
To disable it was just a matter to install the package “xkbset” and to add the following line into ~/.zshrc.
xkbset exp -accessx