Yep. It is probably CPU and disk intensive (on the DB server). I haven't had much time to look into it yet
We can't use it unless it really scales well, so it either scales well now, or it'll improve over the 1.5.x series as we learn about it.
(Once we migrate our large installs to 1.5 we'll have really juicy performance profiling data. There are some really interesting database profiling scripts Penny's put together that read the database logs and tell you all sorts of interesting things about what to optimize ;)