efficienct improving on sorting - mongodb

Question

My code is here:
https://github.com/Dyfused/ExplodeX/blob/master/labyrinth-mongodb/src/main/kotlin/explode2/labyrinth/mongo/LabyrinthMongo.kt#L87,L259
document structures
Set example:
{
"musicName": "Ignotus Afterburn",
"musicComposer": "Arcaea Sound Team",
"introduction": "yay",
"coinPrice": 0,
"noterName": "official",
"noterUserId": "f6fe9c4d-98e6-450a-937c-d64848eacc40",
"chartIds": [
"l54uw6y79g1pspqcsvok31ga"
],
"publishTime": {
"$date": {
"$numberLong": "1640966400000"
}
},
"category": 0,
"hidden": false,
"reviewing": false
}
GameRecord example:
{
"playerId": "92623248-b291-430a-81c5-d1175308f902",
"playedChartId": "qsx8ky1c94f9ez1b8rssbsr2",
"score": 994486,
"detail": {
"perfect": 1308,
"good": 7,
"miss": 1
},
"uploadTime": {
"$date": {
"$numberLong": "1"
}
},
"r": 0
}
Program logic
When the sort type is 'DESCENDING_BY_PLAY_COUNT', it just stuck and return something seemed to be wrong.
Line 101 to 234 is just filtering, it have nothing to do with sorting.
I want to sort the sets by the related records count. So I firstly 'lookup' the related records then get the size of the related records. But it seemed has a heavy efficient problem, which I cannot figure out how to resolve.
Relevant pipeline additions:
SearchSort.DESCENDING_BY_PLAY_COUNT -> {
pipeline += lookup("GameRecords", "chartIds", "playedChartId", "playRecords")
pipeline += addFields(Field(
"playCount",
MongoOperator.size.from("\$playRecords")
))
pipeline += sort(descending(SongSetWithPlayCount::playCount))
}