We're working on the cardinality problem. Will be resolved in an upcoming release. Moving the index over to a disk based format that will hopefully still be fast and not sacrifice lookup performance.
You want to keep the amount of different data that you are indexing/tagging on low. As an example with my situation, I was tracking what could be amounted to connections between nodes in a very large tree. I had a lot of distinct pairs, which means that I had a high cardinality. When the cardinality increases a query that used to take a millisecond to load could move to a couple seconds.