Painter speed problem after using impasto brushes

Hi Jinny and all,

Working on a MacPro 9GB of Ram and Leopard, I see a problem that seems affect both PX.I and P11:

on a file of 4000x4000 pixel at 250dpi (but it happens on low res document) I painted with different brush categories at big size:

Chalk (Square chalk size 183, Square chalk 35 size 353,5), Pens (Real drippy pen) and  (Wet Brush size 382,6). They works well and fast

 

Then I choose  artist's oil- impasto oil size 322 and after I tried to paint with the square chalk used before: unusable.

It became really slow, also at small sizes.

I cleared impasto but nothing change and I have to close the document.

Reopening the same image Chalks and the other brushes work fine until I used impasto oil or impasto brushes again.

I'm going to check it with other brushes but  can someone verify this behavior?

thanks a lot

Paolo