I noticed that the Overview Docker has very bad quality for showing preview of the canvas. It's very well noticeable while doing some line art and using small window of the Overview Docker.

Here it's a discussion from IRC about it:
[17:40:19] <boud> actual code is in KisPaintDevice:: createThumbnailDevice
[17:40:56] <eliotJ> boud: Does better scaling mode give far worse performance ?
[17:40:58] <boud> I think at one point we took a qimage of the whole image and scaled it using qt -- that used oodles of memory and was very slow
[17:41:25] <boud> eliotJ: well, I'm actually wondering whether we ever tried scaling using Krita's scaling method.
[17:41:36] <boud> But that's _also_ the wrong approach
[17:41:58] <boud> we should just make the overview widget a canvas and let opengl do the scaling, instead of getting a thumnbail from the image
[17:42:45] <boud> and it should be a canvas that shares the tiles with the main canvas, of course

[18:23:55] <eliotJ> boud: I have another (temporairly solution maybe), better scaling mode and update only after finished the stroke or after, for example, 500ms... Imho It doesn't need real time update inside these Overview.
[18:26:13] <eliotJ> boud: Maybe these solution is easy to implement and sufficient.
[18:27:49] <eliotJ> It's just Overview - and this way we doesn't lost good performance and get good quality of thumbnail.
[18:56:54] <boud> eliotJ: well... that's going to give big problems with memory usage.

Reproducible: Always