All nodes look pretty idle except, strangely, elasticsearch-elasticsearch-data-1b-12 which appears to have a number of threads busy writing to replicas. It is a bit suspicious that this one node is busy (but only a bit, I don't think it's worth following up right now).
So I agree with what Christian said, the bottleneck appears to be outside the cluster. Try pushing it harder!
I also note you're using the third-party ReadOnlyREST plugin. That isn't obviously a problem for you, but it's been observed to cause problems for others. This is another reason to upgrade: security is included in the default (cost-free) basic license from 6.8 onwards so there's no need to rely on third parties for this feature any more.