I have a synonyms file of around 1GB. The index has around 25 million documents. When the index is closed and then opened again, it takes almost 3 hours for the shards both primary and replicas to be allocated.
Number of primary shards: 7
Number of replica shards: 14
Average shard size: 20GB.
Output of hot_threads is as given https://pastebin.com/ipTVjd8A