Rails 4 made asset pipeline precompiling now lightning fast compared to rails 3.x. There were two reasons (that I am aware of) for the slowness in rails 3:
- it would regenerate all the assets, each time (you can fix this via the turbo-sprockets-rails3 gem).
- it used to precompile everything twice: once with the digests (the checksums in the filename) and once without.
The precompilation happened twice because your assets can reference other assets (eg a stylesheet with some image or font urls) so the difference between the digest assets and the non digest assets wasn’t just the filename.