For improving general-purpose gzip / zlib compression, there is the Zopfli project [1] [2]. It also has (alpha quality) code for PNG file format; since this functionality wasn't originally included, there are also third-party projects [3].

You might be able to shave a percent or so off the download size of compressed assets.

[1] https://news.ycombinator.com/item?id=5316595

[2] https://news.ycombinator.com/item?id=5301688

[3] https://github.com/subzey/zopfli-png