just to add my 0.02 here as I happened on this and I have avoided using gzip stuff from nim as it is too slow. Mark (hi!), I know you linked to [hts-nim]([https://github.com/brentp/hts-nim](https://github.com/brentp/hts-nim)) but that will give you multi-threaded decompression for bgzipped files and a VCF parser. If you want to parse the VCF yourself, you can use [hts_lines]([https://github.com/brentp/hts-nim/blob/master/src/hts/files.nim#L38](https://github.com/brentp/hts-nim/blob/master/src/hts/files.nim#L38)) which will do multi-threaded decompression and give you an iterator of lines.
I know this side-steps your original problem and it would be great to see the Stream stuff faster in nim, but, since you probably have htslib on your system, you can use hts-nim. I thought about wrapping zlib.h or bgzip.h directly with a lines iterator in its own module but havent done so yet.
