Elasticsearch replication performance is slow

I have an elasticsearch setup that had until now one machine (16core 64GB RAM 4x800GB SSD Ubuntu 16.04 Xenial, containing 1.5TB of log data in 450 indexes, running ES 5.1.1.

Now I added a second, identical server to the cluster. Both are connected via a 10GBit network.

All indexes have 1 shard, and I have configured them to have 1 replica after the second server went online.

Now, replicas are being created, but only slowly. Load on both machines is below 1 and IO rates are at about 2MB/s or less.

I am running the following settings:

    "persistent": {
        "cluster": {
            "routing": {
                "allocation": {
                    "node_concurrent_incoming_recoveries": "20",
                    "node_initial_primaries_recoveries": "8",
                    "node_concurrent_outgoing_recoveries": "20"
        "indices": {
            "recovery": {
                "max_bytes_per_sec": "400mb"
            "store": {
                "throttle": {
                    "type": "none"
    "transient": {
        "logger": {
            "org": {
                "elasticsearch": {
                    "indices": "DEBUG"

indices.store.throttle.type does not seem to exist anymore in ES 5.

Why is this so slow?
I could not find any other settings or throttles.

Is there anything else to look at?

What version exactly?
What OS?

ES 5.1.1 on Ubuntu 16.04 Xenial

Does anybody have an idea?

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