I edit:
speed_by_over = 100, // edit this value

to different value but i dont' see a great change (just for 550, where it's very slow...)

So what's value type ?

Re: @decembre:

Set these 3 values like this and it will speed up the scroll when you mouse over the arrows

speed_by_click = .05, // edit this value
speed_by_over = .05,  // edit this value
// z-index
zIindex = 1;       // edit this value

CORRECTION ON THAT PREVIOUS MESSAGE

Leave the numbers as they are by default, except for the speed_by_over number which if you set to something smaller than 10, will help it scroll reasonably faster than it originally does

Thank you for the answer !