On a small performance comparison using the query

[
  {$project: {
    f0: {$setUnion: ["$f0","$f1"]},
    f1: {$setUnion: ["$f0","$f1"]},
    …, 
    f19: {$setUnion: ["$f0","$f1"]}}
  },
  {$project: {f0: 1}}
]

classic needs ~20,000 msecs while SBE needs ~8,000 msecs.

In classic engine, setUnion is implemented using an std::set.
In SBE engine, setUnion is implemented using a flat_hash_set

Replacing set::set with flat_hash_set in classic should close this gap.