development env: is the asset pipeline expected to compile all assets or just those in demand?

I'm trying to understand if, in development, the asset pipeline is supposed to compile ALL assets found in manifest.js or just the ones that are requested.

I've been doing some testing and have found that it appears all assets are compiled even when only a singular small asset is requested. This for us can be extremely slow.

It would seem to me that all assets are compiled even if a singular, small, asset is requested. This, for us, is very slow.