Why are there so many slow `GETMORE` operations?

[llk2yq]

First, thanks for your work~

Here is my config:

monstache.toml:

[[mapping]]
namespace = "db.good"
index = "good"

[[mapping]]
namespace = "db.goodview"
index = "good"

[[relate]]
namespace = "db.good"
with-namespace = "db.goodview"
keep-src = false

[[relate]]
namespace = "db.book"
with-namespace = "db.good"
src-field = "_id"
match-field = "book"
keep-src = false

mongodb log:

{
    "type": "op",
    "host": "my-server:27017",
    "desc": "conn3074214",
    "connectionId": 3074214,
    "client": "10.0.0.4:46574",
    "appName": "monstache",
    "clientMetadata": {
        "application": {
            "name": "monstache"
        },
        "driver": {
            "name": "mongo-go-driver",
            "version": "1.17.2"
        },
        "os": {
            "type": "linux",
            "architecture": "amd64"
        },
        "platform": "go1.23.6"
    },
    "active": "true",
    "currentOpTime": "2025-10-12T13:08:13.179+08:00",
    "effectiveUsers": [
        {
            "user": "pub",
            "db": "dev"
        }
    ],
    "isFromUserConnection": true,
    "threaded": true,
    "opid": -873179958,
    "lsid": {
        "id": "73fd300e-bdde-4a1d-9b0b-a03a0138a0d2",
        "uid": "nVbog0YYoqNxjM6xiBTcVaYasFZhGBl7ILdtGLH/IEQ="
    },
    "secs_running": 33,  // very slow
    "microsecs_running": 33396393,
    "op": "getmore",
    "ns": "db.goodview",
    "redacted": false,
    "command": {
        "getMore": {
            "low": -1512137024,
            "high": 640742422,
            "unsigned": false
        },
        "collection": "goodview",
        "lsid": {
            "id": "73fd300e-bdde-4a1d-9b0b-a03a0138a0d2"
        },
        "$clusterTime": {
            "clusterTime": {
                "$timestamp": "7560197538330968081"
            },
            "signature": {
                "hash": "4RKigA4L9enGvuqfbwzDirjwYHQ=",
                "keyId": {
                    "low": 47,
                    "high": 1750362083,
                    "unsigned": false
                }
            }
        },
        "$db": "db"
    },
    "queryFramework": "classic",
    "planSummary": "COLLSCAN",
    "cursor": {
        "cursorId": {
            "low": -1512137024,
            "high": 640742422,
            "unsigned": false
        },
        "createdDate": "2025-10-12T05:01:37.817Z",
        "lastAccessDate": "2025-10-12T05:07:28.376Z",
        "nDocsReturned": 861839,
        "nBatchesReturned": 8,
        "noCursorTimeout": false,
        "tailable": false,
        "awaitData": false,
        "originatingCommand": {
            "aggregate": "good",
            "pipeline": [
                {
                    "$lookup": {
                        "from": "book",
                        "localField": "book",
                        "foreignField": "_id",
                        "as": "book_obj"
                    }
                },
                {
                    "$unwind": {
                        "path": "$book_obj",
                        "preserveNullAndEmptyArrays": true
                    }
                },
                {
                    "$project": {
                        "book": 1,
                        "book_obj": 1,
                        "stock": 1,
                        "_id": 1
                    }
                }
            ],
            "cursor": {
                "batchSize": 101
            },
            "collation": {
                "locale": "simple"
            },
            "querySettings": {}
        },
        "operationUsingCursorId": 3421787338
    },
    "numYields": 1094,
    "queues": {
        "ingress": {
            "admissions": 1,
            "totalTimeQueuedMicros": 0
        },
        "execution": {
            "admissions": 78846,
            "totalTimeQueuedMicros": 361532
        }
    },
    "currentQueue": null,
    "locks": {
        "Global": "r"
    },
    "waitingForLock": "false",
    "lockStats": {
        "Global": {
            "acquireCount": {
                "r": 78846
            }
        }
    },
    "waitingForFlowControl": false,
    "flowControlStats": {}
}

db.good has 2200000 docs, and db.book has 3200000 docs;

Why are there so many slow GETMORE operations? And how to optimize it?