1. You have to optimized the XMill algorithm so that the compression file size should be reduced with minimized the compression or decompression time.
As per my finding for this change you have to modify the xcmill for optimized the compression process and xdmill project to optimized the decompression process.
2. If time permits then please add one more algorithm that is 7Zip algorithm option to compresses the XML data. Right now this algorithm supports GZip, Bzip2, PPM algorithm for compressing the data. I will provide you the 7Zip dll or links for implementing this option.