I understand that external dependencies within a configuration are downloaded in parallel when resolved. However, what happens with dependencies created using different detached configurations? Is there a way to ensure that dependencies, regardless of the scope or configuration they’re defined in, are all downloaded in parallel when resolved?
How things are resolved depends on quite some details.
For example for normal configurations when using configuration cache, currently all dependencies are only downloaded sequentially as the configuration phase is single threaded, and also all other resolution at configuration phase should be sequentially. This is also afaik the main reason builds could be slower with CC enabled, depending on the build.
If you resolve one detached configuration at execution phase and wait for it to finish and then resolve the next, of course it is done configuration for configuration, unless you do that in parallel if that is possible. 