That is easy. Your encoder speed is wrong.
/* Normally cou want a next/previous actions with every click of your encoder.
Unfortunately, the encoder have a different count of phase changes between clicks.
Select an encoder speed from 0 = fastest to 2 = slowest that results in one menu move per click.
*/
#define UI_ENCODER_SPEED 1
so increase the value by 1 that you have and it should then have the correct speed.