I agree. What we saw is exponential growth in the time it takes to load global ordinals for high cardinality fields, so 30M docs with a field with 30M unique values might take 30s but 70M will take much longer.
The query was a simple terms aggregation on the high cardinality field, with match_all or any other type of query. We can send hot_threads next week.
We can share our full sample data we use for testing (on S3), we can send details privately.