In source code must be some function or variable for that, but I can't find it and I don't know how to even google it
Changing the speed of a keypress is handled at the level of the Xorg server. The xset command is what you are looking for.
The r option controls the autorepeat. Invoking with -r, or r off, will disable autorepeat, whereas r, or r on will enable autorepeat. If the server supports the XFree86-Misc extension, or the XKB extension (which it does), then a parameter of rate is accepted and should be followed by zero, one or two numeric values. The first specifies the delay before autorepeat starts and the second specifies the repeat rate.
For example, I have this command run every time I login (in .xinitrc):
xset r rate 300 50
This increases the autorepeat on holding all keys.