How to find the common ancestor of two branches in SVN?

Imagine you've got a big SVN tree with branches all over the place. There is the trunk, there are branches, those branches have branches, etc. So, given two branches in the tree, how can you find the common ancestor?

I know that you could simply take the full log and compare it, but it get's kinda slow if your trunk has 75,000 revisions (and most of the time you do compare trunk with another, possibly faraway, branch).

The process will be automated, so you can suggest things which are not done easily by hand.

Added: Forgot to say, I need to get it done in real time. Not in "real time" as in 10ms, but in "real time" as in "before the person waiting for the output gets annoyed". So it'd be nice if it was under 10s.