I would not get to know svn:externals if I haven\'t run into the this page. So, I setup my working folder. Then
mkdir lib/vendor
svn add --parents lib/vendo
The svn:externals mechanism is behaving as advertised. The problem is that you are using a public subversion repository (accessed over the internet) inside what I assume is a local repository. As a result, you are seeing a significant discrepancy in the time to update the internal repository vs. the externally referenced one.
Copying to your repository is certainly a viable solution. How to do this is explicitly documented in the Subversion book.