As I mentioned in another comment, the minigzip used is very suboptimal due to reading and decompressing only 1 byte per inflate() call. Please retest with --zlib-compat or the fix that was merged today.
Unfortunately the minigzip implementation used is using a very minimal codebase to implement the functionality, among these is a problem where it will read and decompress 1 byte per call to inflate(). A patch and pull request for this has been available since yesterday at least, and I merged this today.
In stock zlib this codepath is only used if you defined Z_SOLO, but in zlib-ng it is the default unless you run configure with --zlib-compat (or otherwise set the correct defines).
Please retest with the updated commit, or even better with --zlib-compat if you want to get close to an apples-to-apples comparison.
EDIT:
PS: This goes to show that minigzip can not really be used as a reliable benchmark, since actual applications linking to the library could be using it differently (for better or worse).
Also #zlib-ng on freenode if you or anyone else want to discuss.