[This issue is similar to https://bugs.kde.org/show_bug.cgi?id=372221, but instead of relating to the pop-up palette, this has to do with the "Select a Color" pop-up window.]

The "Select a Color" pop-up window currently feels a lot worse than most other parts of the Krita GUI and doesn't control in a way that's consistent with other similar tools (like the Advanced Color Selector, or the Pop-up Palette's color selector). There seems to be two main reasons for this:

1.) The "Advanced Color Selector" docker has a very nice user-experience feature where the current selecton of it's internal widgets (the hue ring, the saturation/value triangle, as well as the fine HSV adjustment sliders towards the bottom) tries to follow the position of the cursor, even when the cursor leaves the widget area or the docker altogether. 

This type of behavior feels great and can be found all over Krita. However, the "Select a color" pop-up window (that appears when you click on your current foreground/background colors) currently doesn't seem to have this behavior and, as such, feels a lot worse. As it is right now, the instant your cursor leaves the widget area (for example the hue ring) the selection stops following. The result is that the controls in the "Select a Color" window feel 'sticky' and it can actually be difficult to move the selection to the precise place where you want it (especially in the corners of the saturation/value triangle).

2.) The second issue is that the "Select a Color" window seems to lag a little bit, or rather, the GUI seems to refresh/update less frequently. This further adds to the rough and choppy feeling.

https://drive.google.com/file/d/1VUqsZdGeJQabd-bFUegM-5ig9DqvLnOu/view?usp=sharing

Linked above is a video clip showing the difference in behavior/performance between the "Advanced Color Picker" and the "Select a Color" window.