We've seen similar degradation. Here the major slowdown was caused by parent-child relationships (defined in the mapping, but not used). Once we rebuilt the indexes without PC, average response times returned to the same interval like in 2.4, but still there are high peaks, which weren't in 2.4.
Since the index was created by restoring the old ES 2.4 snapshot, you can see the "string" instead of "text" in mapping.
For some fields like Title, Description, we have multiple language specific fields to better support multi-lingual query, like Title_english, Title_german, etc.
We have nested fields to be used in nested function_score with groovy script_score:
After investigation, the degradation might not be due to 5.2 migration. I am still investigating the root cause, and will get back to you with details.
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