This suggests your master node is having trouble processing cluster state updates caused by the simultaneous creation of a bunch of indices. Is it working its way through these tasks at all? In other words, if you GET /_cluster/pending_tasks repeatedly do you see the list shrinking over time?
Some possible causes:
-
Too many shards in your cluster. Every time you create an index the master must allocate its shards somewhere, and it takes into account the locations of all the other shards when doing so. The more shards you have, the longer this calculation can take.
-
Overloaded master. Similar to the previous point, if the master is busy doing non-master things then it has fewer resources available to process cluster state updates quickly.
-
Failing/slow node. Cluster state updates happen in order, with each update waiting for all nodes to acknowledge before moving onto the next one, so a failing node can slow this down until it's removed from the cluster. The master logs should indicate if there's a node that is not acknowledging updates fast enough.