Experiment:

Collection0 contains 10,000 docs of the form:

> db.Collection0.find().limit(3)
{ "_id" : ObjectId("65badbe9a940789d3800c3c1"), "arr" : [ 5, 6, 7, 0, 4 ] }
{ "_id" : ObjectId("65badbe9a940789d3800c3c2"), "arr" : [ 8, 4, 3, 7, 0 ] }
{ "_id" : ObjectId("65badbe9a940789d3800c3c3"), "arr" : [ 4, 6, 5, 5, 0 ] }

Query:

db.Collection0.aggregate([
   {"$unwind": {"path": "$arr"}},
   {"$group": {"_id": "$arr"}}
]);

Per explain execution stats, this runs in

• 58 msec - Classic
• 642 msec (11x as long) - SBE

Individual stage time estimates (e) and total actual times from db.Collection0.explain('executionStats'), all in milliseconds:

Classic millisecs per stage

SBE millisecs per stage

I will add Genny-based repro instructions in a comment.