How timely - I, too, got frustrated by the fact that org-babel src block evaluation is synchronous, so I built ob-async [1]. Rather than target a specific babel language, it aims to support any flavor of src block. It uses jwiegley's async [2] to execute the block in a second Emacs process and then inserts the results in the buffer when complete. There are still some rough edges (no :session support, for example), but it's worked well for my somewhat limited use cases so far.

[1] https://github.com/astahlman/ob-async [2] https://github.com/jwiegley/emacs-async

You don't necessarily have to push all files into the global agenda. Instead, you can set `org-agenda-custom-commands` to something like

  '(("p" "Some project"
     ((alltodo ""))
     ((org-agenda-files '("path/to/file.org"
                          "path/to/another.org"))))
    ("o" "Another project"
     ((alltodo ""))
     ((org-agenda-files '("another/project/file.org")))))

Now using `M-x org-agenda p` will give you a todo list for the first project, and `M-x org-agenda o` for the second project.

The current and recent versions of Emacs ship with org-mode and the necessary support [1]. The 'org' package in melpa/elpa is likely to be obsolete and installing it is likely to create versioning issues between various elisp files.

[1]: http://orgmode.org/manual/Installation.html

https://news.ycombinator.com/item?id=12429437 was related.