After switching to the toolchain v5 as default in 341832a we've observed few performance regressions including a 10% regression in ElemMatchLargeMixedInAndOrWithDuplicates causing BF-37502.

The benchmark in question executes a simple find query with nested $elemMatch, $or, and $in predicates. Both $or and $in predicates have many arguments. The benchmark targets SBE engine (not yet enabled by default) which uses absl::raw_hash_set for $in predicate implementation.

Our initial investigation identified >9% regression in absl::raw_hash_set::find(). It would be nice to 'recover' this performance on toolchain v5, however, we should consider the best way to do that since absl is a third-party library.