Where is the benchmark code for this? Cassandra and MongoDB are not key-value stores, you should have compared Memcache and Redis for this. It's like comparing MySQL and Memcache for your key-store corner case and be surprised that it's 20x slower! Ridiculous
Comparing Hyperdex against Cassandra and MongoDB is appropriate. All three are distributed databases with secondary indexes and an emphasis on consistency and scalability.
The other two databases you mentioned are not in the same problem domain. Redis will not manage more data than it can address in memory (barring the deprecated virtual memory paging) and Memcached doesn't bother with persistence at all. Both of these are very much memory databases targeting ephemeral storage tasks. Neither of them have distributed capabilities beyond very minimal master/slave configurations.
The YCSB benchmarks [0] were developed for Yahoo to evaluate a wide range of NoSQL databases by abstracting a few common operations to measure the primitives shared by all of them.
