By default Rethink requires an fsync from disk to guarantee data safety. So a typical default query goes like this: client request -> network trip -> wait on disk fsync -> network trip -> ack. That's going to be really slow in any system, and makes 10k writes/sec pretty much impossible on any rotational disk.

In Rethink you can turn on soft durability which will allow buffering writes in memory. That would drastically speed up write performance. Another option is to use concurrent clients. Rethink is optimized for throughput and concurrency, so if you use multiple concurrent clients the latency won't scale linearly.