Since when is git-svn's speed relevant for evaluating git? If nothing else, you import only once, then use the destination VCS for. the rest of the project's life (or until something better comes along :-p ).
Importing into git can be _fast_ and is limited (on the git side) by the write speed of your disk, and your CPU's zip and sha computing.
On the other hand, last time I checked git-svn essentially does a HTTP request for every revision it pulls. That's hardly fast under the best circumstances. Sorry to say, I have zero interest in finding out whether this could be sped up.