$in lists are being sorted and de-duplicated before serialization.

The sorting and de-duplicating of large $in lists has significantly improved the execution times of those queries, however for query stats it has made serialization much more expensive leading to regressions of such queries when compared against the baseline.

large $in query without query stats

large $in query with query stats

The regression (pertaining to query stats) was introduced by https://jira.mongodb.org/browse/SERVER-76904, and specifically this line.