Currently we create a new CollectionAcquisition for each update here:
https://github.com/mongodb/mongo/blob/14490ae6797f5d1adb7424167d9a0d2168dd131d/src/mongo/db/ops/write_ops_exec.cpp#L1350

When in theory we can re-use a single one for all (or at least a sub-batch of) updates in a single performUpdates call.
https://github.com/mongodb/mongo/blob/14490ae6797f5d1adb7424167d9a0d2168dd131d/src/mongo/db/ops/write_ops_exec.cpp#L1580

This may lead to significant performance improvements for operations like $merge.