Poor performance at large index size

I am using ES v2.4.0 for a full text query load on webpage content.
When I have an index of 15 GB (small mapping, only content is analyzed field), then the search performance is 2000 QPS. But on another index of size 1TB residing on the same machine, I experience very poor performance (around 60 QPS).
My configuration is 3 AWS instances (16G ram each, 8 cores, SSDs) which are data nodes.
The 15 GB index has 3 primary shards, but the 1 TB index has 5 shards (the default value, which doesnt balance equally between the 3 nodes).
Also, the 1 TB index has all fields as analyzed, whereas the 15 GB one has just the content field analyzed.

What could be the reason behind 1 TB index's extremely low performance?

Thanks a lot!

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