While it's higher data density with Zopfli, Google claims that only the compression speed of this new method is slower but that file decompression speeds are unaffected. While the compression is "a lot slower", Zopfli is said to be 3~8% smaller than Zlib at maximum compression.
Another limitation that may prevent some areas where Zopfli is worthwhile is that aside from the much longer compression time, it usues about 2~3x more CPU time than zlib. Google engineers recommend Zopfli for cases like sending static content over the network. More details on Zopfli compression can be found via the Google Open-Source Blog.