I opened a PR #6973 this morning for Zlib Block Compression for block
relay and at the request of @sipa  this should have a BIP associated
with it.   The idea is simple, to compress the datastream before
sending, initially for blocks only but it could theoretically be done
for transactions as well.  Initial results show an average of 20% block
compression and taking 90 milliseconds for a full block (on a very slow
laptop) to compress.  The savings will be mostly in terms of less
bandwidth used, but I would expect there to be a small performance gain
during the transmission of the blocks particularly where network latency
is higher.