push.replication.compression-algorithm (collection.cfg setting)
The compression algorithm to use when transferring compressible files to Push slaves. The following options are available:
Gives the highest compression but is the slowest. Should be used when a slaves network bandwidth no more than 100Mbps
Gives good compression but still uses some CPU on the master.
If 42 push slaves are all sharing the same 1Gbps link to master than GZIP might be used to save network resources: