Improve performance of decomposedfs

[aduffeck]

This PR improves the performance of the decomposedfs using the following measures:

• Reuse exisiting space and parent information when iterating over a list of child entries
• Do not resolve the parent node just to transform the parent id
• Gather and transform the child information concurrently

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.