Krita gets extremely slow while handling trnasparent .png files

[Fabien NGUYEN]

I usually work on 4000 x 4000px @ 400 dpi files and I happen to insert .png files in my document by :

+ importing the layer from an imported .png file in Krita ;
+ pasting from The GIMP as Monitor.

Some parts of the layer are transparent.

Every time I handle this layer and work on the layer itself (rotate, screw, resize), Krita freezes for a while when I try to undo one of this operations, for several minutes sometimes. No apparent disk access during this undo.

Memory handle problem ?

Krita 2.4.0
Koffice 2.3.2
Kubuntu 12.04