Generally with graphics apps, the more layers you add, the more memory / resources are required. As we also need to take into account elements like paint thickness, wetness, lighting etc for each layer, we have a bit more to deal with.
The file you're working with is pretty large ( 3600 x 3300 ) so I'd expect to see some slowdown as more layers are added.
We're looking at making some changes in the future that may speed up the application in this kind of situation. In the meantime, turning lighting off ( F5 ) temporarily may speed up some processes such as painting for you.
Resident Bug-Hunter and Support Guy