Hi Stephen,
I've released version 1.1 which is definitely faster, it can export your test file in about 1 second with 1.3 MB of size.

The problem was not the compression algorithm, it was the way the PNG encoder from FreeHEP reads the pixels from the source image (a very inefficient way), so I have replaced the critical code with a most efficient one.

I've aso changed the default compression level from 9 to 5, although it doesn't much difference.

In regards to the repeating pattern you're right, unfortunately fixing it would require a huge work and I don't think is worth the effort.