One the main things items that are searched on this blog is RotoPaint problems.

In Nuke 6.3 the RotoPaint is painfully slow.

Here are 3 tips that will make Paint work much, much better.

Paint on 1 frame then frame hold that frame. Setting it this way Nuke has to only render one frame of paint instead of all.
Limit the amount of brush strokes to 100. When you get to one hundred start a new RotoPaint node. This will also keep you from losing less if a paint node goes belly up.
Place a shuffle before your RotoPaint node And set the alpha to 1[White]. This will help make he onion skinning work.

Remember the RotoPaint node is very very temperamental. Stay calm.

I'd like to be clear. I did not figure any of this out. These were told to me by other compositors.