I recently added a binary type field to all documents with mapping "store": 
"true".  The field contents are large and as a result the on-disk index 
size rose by 3x, from 2.5Gb/shard to ~8Gb/shard.  

After this change I've seen a big jump in query latency.  Searches which 
previously took 40-60ms now take 800ms and longer.  This is the case even 
for queries which *don't* return the binary field.
I tried optimizing the index down to max_num_segments=1, but query latency 
remains high.

Is this expected?  Obviously queries returning the new field will take a 
hit (since field data needs to be loaded from disk).  But I would've 
expected other queries should not be much affected.

Is the problem that larger file sizes make memory-mapping and the FS cache 
less efficient?  Or are stored fields still getting loaded from disk even 
when not included in the "fields" term?

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