I don't think they're avoiding Cassandra as their tweet store because it's not capable of performing well. It's probably more cause it would be a really long process migrating every tweet into Cassandra. I'm sure if they could, they would go back to the first day Twitter launched, and use Cassandra as the tweets store.

As for query time, maybe your problem was I/O throughput? I've read Cassandra can be really slow if you have poor I/O throughput. This happens a lot in the cloud, where I/O is usually shared.