Fast method to find Subversion externals using SVNLOOK

Question

I have a large repo, over 100GB, that I want to search for external property values.

I've used a method similar to the one discussed here:

List all paths in subversion repository with a certain property & value

The problem is that it is very slow to traverse the whole repo and get the values.

Does svn store properties per path in a cache somewhere is or there a faster way of searching a repository?

I'm not looking for a product like Fisheye just something I can script.