When holding down a key switch on the keyboard, the key press will repeat after a short delay. The delay from when the key is pressed until the key press is repeated is called the Keyboard Repeat Delay. The rate at which key presses repeats when a button is held down is called the Keyboard Repeat Rate.

Systems Settings in KDE does not allow me to set Keyboard Repeat Rate to more than 50 repeats per second.

This is way too slow for me.

This setting is stored in the file ~/.config/kcminputrc

In this file, you will find a section such as this:

[Keyboard]
KeyboardRepeating=0
NumLock=2
RepeatDelay=300
RepeatRate=75

Here you can just set RepeatRate to whatever you like. I tend to go with a RepeatRate of 75 and a RepeatDelay of 300.