HyperDex provides strongly consistent GET/PUT operations and fast searches over data. It uses a new object placement technique to facilitate these searches, and as a result, a search may contact a very small subset of hosts in the system.
Record insertion affects all replicas of the data, so latency increases with replication.
The first graph on the performance page (http://hyperdex.org/performance/) shows that a 100% insertion workload will exhibit higher latency (and thus lower throughput for a fixed number of synchronous clients). Write-mostly workloads are slower, but I wouldn't say "extremely slow."