the problem here indeed is that you are using 'benchmarking' tools which are not really designed for benchmarking distributed object storage systems. If you want to figure out what your cluster can do, then look at something like HCIBench instead. That will give you a better view.
In your case you are saying a performance impact simply because half of the IO will need to go across the network, and there's a cost to that. You compare it to RAID-0, which has no protection whatsoever.