Does Bazel download artifacts in parallel with the build?

[Konstantin]

In my understanding Bazel downloads external workspaces lazily and in parallel with analysis and even execution of the targes which don't need those workspaces. 

Confusingly console output from hundreds of builds makes an impression that FIRST all externals are fetched and only THEN analysis/execution starts.

Can somebody please shed some light on it?

Do we loose anything if we move fetches to the separate pre-build step for the sake of better time accounting?

Konstantin