Why Nested Aggregations do not perform well

I have been trying a couple of queries which includes nested aggregations and filteredAggregations with subAggregation. But I have heard the fact that try to reduce nesting in aggregation since they degrade the performance. I have searched but haven't got any reason for the same. Can someone explain what happens internally when nested aggregation is used?
I know aggregation uses doc_values. For nested aggregations are they joined with themselves to get results OR how lucene sees these queries. Any help would be appreciated.

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