Indexing rate drops periodically - performance troubleshooting

Elastic Stack Elasticsearch
1

I have a ES 7.9.2, [ 10node cluster each: 4Core, 32G RAM, 500GB SSD]
I tried performance test on index , 5 primary shards, 0 replica.
It reaches around 200K events/sec
and I found weird performance drops. Can you help me how to troubleshoot performance drops.
Where should I look to determine the potential cause of the issue?

thank you

See kibana indexing rate on test index

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2

If you have 5 primary shards and no replica shards, only 5 data nodes will be processing indexing requests for this index. The periodicity in performance drops could very well be due to merging. Are you monitoring disk I/O and iowait on the data nodes? Is there anything in the Elasticsearch logs around these dips?