In other professionell applications (and also my own) I’m used to use a keyboard-hotkey or/and the scroll-wheel feature of my mouse/trackpad to increase and decrease values faster.
So I would recommend that you maybe could consider, to add support for…
Hotkey: Modifier Button + Plus/Minus-Key
…on decreasing/increasing UI-Elements:
- FontSize-Slider Image-Placeholder (new users are restricted to 5 image )