Currently P21 GUI has sliders that are best used when clicking along the slider rather than sliding the sliders...for example changing layer opacity. Not intuitive for a slider mechanism.
Why? because the result of a slider change does not show on screen until the slider has been put to a rest; then the screen updates. Click, wait, click, wait, click....decide, click...
Better, would be screen updates as the slider is moved in real time....because then you can see the changes while you move the slider. User experience is then more intuitive and faster and less work to "home-in" on final desired position.
Yes, implementing live action is a bigger cpu/gpu load, but at a time when everything else is at a standstill and we have plenty of available capacity. Similar to making a brush stroke - resource load when doing something and load near zero when doing nothing.
Most other graphics programs have modernized their GUI to be more reactive/live and Painter could be improved in a same way.