The python startup time always nagged me (17-30ms) and I just searched again for a way to reduce it, when I found this:
The Python-Launcher caches GTK imports and forks new processes to reduce the startup time of python GUI programs.
Python-launcher does not solve my problem directly, but it points into an interesting direction: If you create a small daemon which you can contact via the shell to fork a new instance, you might be able to get rid of your startup time.
To get an example of the possibilities, downl