nixbuild.net supports fetching build dependencies from binary caches. This can make builds a lot faster, especially if your network upload speed is poor.
To activate substitution from binary caches you should use the Nix option builders-use-substitutes.
By default, cache.nixos.org is used when nixbuild.net looks for substitutions, but you can add or remove caches to your account. Currently, there is no UI for editing the list of caches, but if you contact [email protected] you'll be assisted. Also Cachix caches can be added.
Note, that path substitution can happen even though the
builders-use-substitutes option hasn't been activated. If another user has
triggered substitution previously, and your build depends on any of the
substituted paths, they will be used directly. Only paths that come from a
substituter that is trusted by your account will be used. Nix public-key
signatures are used to validate paths downloaded from substituters. Unsigned
paths are not used.