In El Capitan I used Seil and Karabiner for keystroke/repeat/delay finetuning. The Seil/Karabiner combination is no longer working in OSX 10.12 (Sierra). How can I adjust my KeyRepeat and RepeatDelay settings?
Since Seil stopped working in OSX Sierra - Karabiner Elements now inherits some of its functionality - KeyRepeat and delay (InitialKeyRepeat) can't be adjusted to sane values anymore.
After reading issues on github for a while a working solution emerged. The settings involved are:
defaults write -g KeyRepeat -int 1
defaults write -g InitialKeyRepeat -float 8.5
Those settings work good for me. If you want to play around, be sure to reboot to let the new values take effect. Some were saying logout/login is enough, but it wasn't for me.
Also worth noting: In System Preferences -> Accessibility be sure to uncheck the Press & Hold Settings for keyboard. It interferes with the KeyRepeat Setting.
Unrelated note: I had to install Karabiner Elements for simultaneous usage of two keyboards. Sierra reconfigured them so I couldn't use option/alt/shift/command cross keyboard. With Karabiner Elements it is again possible to press shift on the left keyboard and e.g. A on the right keyboard to write a capital A.