When preparing the docDiff for apply we put the insert field in a StringMap: https://github.com/mongodb/mongo/blob/92738c5fa0e8169299e5393e88159b8cbb9559ca/src/mongo/db/update/document_diff_applier.cpp#L106

However, if it is an insert-only workload this map is not used and we just read from the insert vector: https://github.com/mongodb/mongo/blob/92738c5fa0e8169299e5393e88159b8cbb9559ca/src/mongo/db/update/document_diff_applier.cpp#L495-L504

This should cut a StringMap insert for every measurement in time-series inserts.