OK I was wrong, in that I do not do long strokes so it did not happen for me. Seeing Boban’s reply I tried again and got the same effect.
Working slower and in shorter actions seems to cope better.
Research found as FedPete says, there has been this reported as a problem. But is considered resolved in the developer threads I saw, though it clearly is not.
While browsing through the navigation bar (N) you’ll find the “Advanced” subsection, and in there you need to uncheck “Anti-Aliasing”.
It did the trick for me, i can now paint hi res with the biggest brush and it doesn’t lag anymore, so i thought i would share this here in hope it could help others.
More about it here with some supposed help/solutions. I tried some and they made little quantifiable difference.
So I guess it is just a matter of working slower and smaller rather than fast and over big areas.