If/When you upgrade to 9.1, can you try this again and share if it is faster. There are some LOD calc perf improvements in 9.1 that will collapse what were multiple subqueries in 9.0 into one if they happen at the same level of detail.
thank you so much for the reply. This is really good to know! We haven't done anything yet with 9.1 but the potential of faster response times may lead us to accelerate our timeline. Email me (firstname.lastname@example.org) if you want to have a look at the 2 pager performance writeup.