I wrote a blog post about improving the performance of import-from-derivation and other evaluation blocking in Nix. Ever wonder why your Nix builds are sitting there thinking for a long time without actually building something?
The C++ Nix implementation will block during evaluation in an inconveniently serial fashion if things like import-from-derivation or builtin fetchers are used, serializing builds in a nasty way.
In this post I go over the causes of this issue and the way that I rearchitected the callHackage function in nixpkgs to eliminate blocking on import-from-derivation, taking several minutes of frustrating blocking down to just a few seconds.