git-subtree split O(N)

I'm using git-subtree split to extract changes to a directory previously added with git-subtree add, in my local repository. The split is going into its own branch, which I then push to the upstream.

The problem is that every time I split, git-subtree starts from the very beginning of the commit log and works its way forward. I thought that it was only supposed to do that once, and that subsequent splits would start from the last commit in the destination branch (I keep the split branch around).

What's going on?