SUMMARY

the outline shape in preview outline mode shows a very complicated outline for auto brushes that have a low density setting. it results in a slower cursor on canvas. this looks like a new behavior, the former was showing a simple shape (ellipse or rectangle) without the density parameter showing on the brush preview outline, which moved at the same speed as the cursor.
a half workaround is to uncheck "show outline" while painting, which at least lets paint at normal speed.

STEPS TO REPRODUCE
1. set settings > configure > general > cursor >outline shape in preview outline
2. use a pixel brush with auto shape and low density
3. move the very complicated cursor on the canvas

OBSERVED RESULT
the cursor is slow and complicated

EXPECTED RESULT
the previous behavior was showing only the brush outline without the noise of low density, and was much quicker

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: xubuntu 20.04
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: i don't know

ADDITIONAL INFORMATION

my machine is pretty old (about 10 years) so maybe it's something not as noticeable with modern hardware, but the previous behavior worked great and was sufficient for low density brushes.