Check out formula : Encoded array key too long (> 30978). Skipped
Index key size can impact indexer performance. cc @deepkaran.salooja .
If the query is not covered it can impact query performance/cluster scalability too. Example: If you have ~1000 messages for ~1000 recipients, i.e. gives big document in KBs also ANY clause is looping construct i.e loop of n*n (i.e 1Million loops) If the value looking at the front it is good if at the end Filter evaluation needs to go through loops to validate and it can take time.
Arrays can work small size if bigger and nested may be join might be better.