The more fundamental issue that caused BF-28050 is that we repeatedly parse and optimize the $lookup inner pipeline for each input document, not parameterizing the inner pipeline.

The classic $lookup would be used in foreseeable future given that the Nebari $lookup (hash lookup) does not support $lookup with inner pipeline and even though the Nebari $lookup supports $lookup with inner pipeline, it may take a while until we can fully switch to Nebari.

Hope we could address this issue sooner or later.