You completely ignored the part where I said "..without giving any details"?
Without this I am left guessing why it affects you and does not affect others.
One guess I have is, cairo is the culprit. You could experiment by copying all the cairo related DLLs from 2.8.2 and putting them onto 2.8.4 and see if you still the performance hit.
Well, i assumed you had read previously the report i made on your Gimp 2.8.4 release thread in which i gave the detail of what was wrong.
And indeed, it's cairo (at least on Win32), because as soon as i moved the libcairo-2.dll
from your ...\Gimp-2.8.2\bin\
into your ...\gimp-painter\bin\
suddenly no more atrociously slow brush, no more extreme cursor lag, it is -as fast and smooth- as your Gimp 2.8.2 was, even the MyPaint brushes are very fast and smooth.
Only problem then is the remaining problem steeno mentionned : the brushes can't be stopped.
But their behaviour is very close to the original MyPaint brushes, definitively potential in making Gimp not only a very good image editor, but a fantastic painting program in the same time.