Hello there, I'm David Tschumperlé, the main developer of G'MIC.
First, I'd like to thanks @null54 for taking the time to port our G'MIC-Qt plug-in for Paint.Net.
Second, I've read all your messages and indeed noticed that the 'Stylize' filter was painfully slow on Windows.
We have investigated this issue these last days with people at pixls.us, and hopefully we should have found what was going on. We were using too much mutex locks/unlocks in the multi-threaded algorithm, which was slowing down everything. I'm working hard right now to fix this issue. I hope I will be able to post a pre-release tomorrow (Friday) with the fixes included.
If you have any questions related to G'MIC, I'd be happy to answer (depending on the amount of free time I have ).
EDIT: The G'MIC-Qt plug-in is a product of the GREYC Laboratory, mainly done by Sébastien Fourey and myself, with the help of many contributors.