PIZ-Compression / Multithreading

Hello Everyone,

i've got two little questions:
I'm developping a c++-tool to convert OpenExr-images from Maya to 
OpenExr-images which are optimized for Nuke (the tool creates a data 
window and converts the images into scanline-mode with piz compression).

Question 1: Why is the creation of a Image with piz-compression so slow 
comparing to the zip-compression? (The input-File is a tiled EXR with 
RLE-Compression). Is there a way to optimize the conversion from RLE to 
PIZ? The conversion is really slow (several minutes for a 2k exr with 10 
layers) so multithreading wouldn't fix the problem.

Question 2: I want to use Multithreading for writing an exr. Reading the 
documentation i thought i just have to set the GlobalThreadCount and 
then start the writePixels-function for every 20 scanlines (for example) 
again and again and the interface would do the thread-stuff 
automatically. But i don't see any effect. The writing takes exactly the 
same time. What is the right way to do that?

Thanks for your replies.

David