As indicated in this BlenderArtists post I have started developing a version of the space tree addon with most of the core calculations performed by C code.
Currently most functionality is available except for apical control and pruning but a 4x speed up (approximately, your mileage may vary) is worth it. This version is at the moment only available for 64bit Windows platforms (because it was difficult enough already to get that running with Visual Studio 2012) and unlike the previous C enhancements there will be no fallback on pure Python.
This version is developed in its own branch, 'all_c' and the installation follows the exact same pattern as for other addons (don't forget to remove any older version). The zipfile is located here. As of May 2, 2014, this is linked against the 3.4 Python library as shipped with the Blender dailt build. If you use a slightly older version of Blender you might want to use the previous commit that was linked against 3.3.
I am thinking about making a Linux version available but that's not high on my priorities. If someone is willing to compile the module however I can include it with the addon. I think it is entirely possible to have a
csca.pyd and a
csca.so side-by-side. Having different .so files for Linux and OSX is something else, that would require tweaking the Python imports based on the OS type.
The current speed improvements only apply to the generation of the tree skeleton. I think the same improvement should be possible for native skinning algorithm. Also, there is yet no optimization of the C code and no multithreading, so we might improve the speed even more.
Another area of interest that I am currently spending time on is creating good leaf textures. Even with a decent flatbed scanner this is not trivial, yet now that spring is in full gear in my hemisphere I have started on cherry, apple and oak leaves.