Search in many indices is too slow

Elastic Stack Elasticsearch
1

I try to search in a index,it only spends 9-20ms,But if i search in mutiple indices,it spends a long time(about 110-240ms. Isn't it parallel？ And when i use the profile API,i find that next_doc spents the most time,how can i solve this problem.

2

How many indices and shards are you querying?

What is the specification of your cluster with respect to nodes, RAM, CPU cores and storage?

Which version are you running?

3

It's about thirty indices and there are 5 primary shards,no replic shards,version is 5.5.3.
20190131212937

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