One of the most requested features, the so-called upstream caches was released today.
It is enabled by default for all caches, and the owner of the binary cache can disable it via Settings.
When you push store paths to Cachix, querying cache.nixos.org adds overhead of multiples of 100ms, but you save storage and possibly minutes for avoiding the pushing of already available paths.
Queries to cache.nixos.org are also cached, so that subsequent push operations do not have the overhead.
In the future you are going to define upstream caches within Cachix so that the typical setup of a private and public cache does not duplicate the entries between the two.
Happy caching! Domen