SERVER-82549 - Here is explain() for the fast query:

{
  "explainVersion": "2",
  "queryPlanner": {
    "namespace": "XXX_production.customers",
    "indexFilterSet": false,
    "parsedQuery": {
      "customer_user_id": {
        "$eq": "6267a29a50102f009b84e9d1"
      }
    },
    "queryHash": "CDDA2482",
    "planCacheKey": "A6431CBA",
    "maxIndexedOrSolutionsReached": false,
    "maxIndexedAndSolutionsReached": false,
    "maxScansToExplodeReached": false,
    "winningPlan": {
      "queryPlan": {
        "stage": "FETCH",
        "planNodeId": 2,
        "inputStage": {
          "stage": "IXSCAN",
          "planNodeId": 1,
          "keyPattern": {
            "customer_user_id": 1
          },
          "indexName": "customer_user_id_1",
          "isMultiKey": false,
          "multiKeyPaths": {
            "customer_user_id": [            ]
          },
          "isUnique": false,
          "isSparse": false,
          "isPartial": false,
          "indexVersion": 1,
          "direction": "forward",
          "indexBounds": {
            "customer_user_id": [
              "[ObjectId('6267a29a50102f009b84e9d1'), ObjectId('6267a29a50102f009b84e9d1')]"
            ]
          }
        }
      },
      "slotBasedPlan": {
        "slots": "$$RESULT=s12 env: { s4 = 1698578154817 (NOW), s2 = Nothing (SEARCH_META), s3 = Timestamp(1698578154, 458) (CLUSTER_TIME), s6 = KS(646267A29A50102F009B84E9D10104), s1 = TimeZoneDatabase(America/North_Dakota/New_Salem...Europe/Rome) (timeZoneDB), s7 = KS(646267A29A50102F009B84E9D1FE04), s11 = {\"customer_user_id\" : 1} }",
        "stages": "[2] nlj inner [] [s5, s8, s9, s10, s11] \n    left \n        [1] cfilter {(exists(s6) && exists(s7))} \n        [1] ixseek s6 s7 s10 s5 s8 s9 [] @\"88869198-6429-48f7-9219-c970de48931f\" @\"customer_user_id_1\" true \n    right \n        [2] limit 1 \n        [2] seek s5 s12 s13 s8 s9 s10 s11 [] @\"88869198-6429-48f7-9219-c970de48931f\" true false \n"
      }
    },
    "rejectedPlans": [    ]
  },
  "executionStats": {
    "executionSuccess": true,
    "nReturned": 2,
    "executionTimeMillis": 0,
    "totalKeysExamined": 2,
    "totalDocsExamined": 2,
    "executionStages": {
      "stage": "nlj",
      "planNodeId": 2,
      "nReturned": 2,
      "executionTimeMillisEstimate": 0,
      "opens": 1,
      "closes": 1,
      "saveState": 0,
      "restoreState": 0,
      "isEOF": 1,
      "totalDocsExamined": 2,
      "totalKeysExamined": 2,
      "collectionScans": 0,
      "collectionSeeks": 2,
      "indexScans": 0,
      "indexSeeks": 1,
      "indexesUsed": [
        "customer_user_id_1"
      ],
      "innerOpens": 2,
      "innerCloses": 1,
      "outerProjects": [      ],
      "outerCorrelated": [
        5,
        8,
        9,
        10,
        11
      ],
      "outerStage": {
        "stage": "cfilter",
        "planNodeId": 1,
        "nReturned": 2,
        "executionTimeMillisEstimate": 0,
        "opens": 1,
        "closes": 1,
        "saveState": 0,
        "restoreState": 0,
        "isEOF": 1,
        "numTested": 1,
        "filter": "(exists(s6) && exists(s7)) ",
        "inputStage": {
          "stage": "ixseek",
          "planNodeId": 1,
          "nReturned": 2,
          "executionTimeMillisEstimate": 0,
          "opens": 1,
          "closes": 1,
          "saveState": 0,
          "restoreState": 0,
          "isEOF": 1,
          "indexName": "customer_user_id_1",
          "keysExamined": 2,
          "seeks": 1,
          "numReads": 3,
          "indexKeySlot": 10,
          "recordIdSlot": 5,
          "snapshotIdSlot": 8,
          "indexIdentSlot": 9,
          "outputSlots": [          ],
          "indexKeysToInclude": "00000000000000000000000000000000",
          "seekKeyLow": "s6 ",
          "seekKeyHigh": "s7 "
        }
      },
      "innerStage": {
        "stage": "limit",
        "planNodeId": 2,
        "nReturned": 2,
        "executionTimeMillisEstimate": 0,
        "opens": 2,
        "closes": 1,
        "saveState": 0,
        "restoreState": 0,
        "isEOF": 1,
        "limit": 1,
        "inputStage": {
          "stage": "seek",
          "planNodeId": 2,
          "nReturned": 2,
          "executionTimeMillisEstimate": 0,
          "opens": 2,
          "closes": 1,
          "saveState": 0,
          "restoreState": 0,
          "isEOF": 0,
          "numReads": 2,
          "recordSlot": 12,
          "recordIdSlot": 13,
          "seekKeySlot": 5,
          "snapshotIdSlot": 8,
          "indexIdentSlot": 9,
          "indexKeySlot": 10,
          "indexKeyPatternSlot": 11,
          "fields": [          ],
          "outputSlots": [          ]
        }
      }
    },
    "allPlansExecution": [    ]
  },
  "command": {
    "find": "customers",
    "filter": {
      "customer_user_id": "6267a29a50102f009b84e9d1"
    },
    "$db": "tablesolution_production"
  },
  "serverInfo": {
    "host": "atlas-tzcwdx-shard-00-01.uhz3j.mongodb.net",
    "port": 27017,
    "version": "7.0.2",
    "gitVersion": "02b3c655e1302209ef046da6ba3ef6749dd0b62a"
  },
  "serverParameters": {
    "internalQueryFacetBufferSizeBytes": 104857600,
    "internalQueryFacetMaxOutputDocSizeBytes": 104857600,
    "internalLookupStageIntermediateDocumentMaxSizeBytes": 104857600,
    "internalDocumentSourceGroupMaxMemoryBytes": 104857600,
    "internalQueryMaxBlockingSortMemoryUsageBytes": 104857600,
    "internalQueryProhibitBlockingMergeOnMongoS": 0,
    "internalQueryMaxAddToSetBytes": 104857600,
    "internalDocumentSourceSetWindowFieldsMaxMemoryBytes": 104857600,
    "internalQueryFrameworkControl": "trySbeEngine"
  },
  "ok": 1.0,
  "$clusterTime": {
    "clusterTime": {
      "$timestamp": {
        "t": 1698578154,
        "i": 458
      }
    },
    "signature": {
      "hash": {
        "$binary": {
          "base64": "ZE1bbCRvoUbme5wRChuqj2iIGVw=",
          "subType": "00"
        }
      },
      "keyId": 7250897925230821726
    }
  },
  "operationTime": {
    "$timestamp": {
      "t": 1698578154,
      "i": 458
    }
  }
}