Paintbrush paints noisy strokes at low sizes

[cactushead]

When I zoom in to 2x zoom and draw with the mouse with the tool options as pictured, the quality of the stroke looks noisy and bad. In the long diagonal stroke, the centreline is noticeably darker in some parts and it makes the width look uneven. It's a little less noticeable at 1x zoom. There are sometimes 'bites' taken out of the line, when there's a light-coloured pixel where I'd expect one of the same grey colour as as its diagonal neighbours along the line. These bites are noticeable even at 1x zoom. As well as these issues, when I draw a nearly vertical line by dragging the mouse down and slightly to the right, it seems as though the brush dabs are jittering from side to side down the length of the stroke. Why do these things happen? In drawing programs such as Krita or Paint Tool SAI, I've not reproduced such issues.

The three dots are just for reference---they are single brush strokes in a pixel corner, a pixel edge, and a pixel centre.

At 4x zoom and showing brush settings:
   

At 1x zoom: