I a blog on compiling the server I found a remark about explicitely setting the compression level to the lowest level of zlib via gzopen(filename, "wb1"). I tested this with my usual test game.
level size time for load/save
1 3664kb 4045/1393ms
6 2985kb 4052/1386ms
9 2882kb 4042/5110ms
default 2975kb 4144/1507ms
One can argue to set it to level six; but the default already does a good job too. Exspecially that at good downlaod rates of 16 MBit (or realistic about one MB per second) the differences between level 1 and default are 200 ms longer compression but 600 ms shorter download.
Anyway, nice to know there is a parameter to play around. (And I am happy to see your impact.)