MongoDB very slow $count after $lookup - mongodb

Question

Help, I am using MongoDB 4.2.6, and writing an aggregate to obtain the number of filtered data from collections with 40000+ data. Before applying the $count method, I need to $lookup an extra collection as well.
Here is my aggregate
db.exams.aggregate([{
$match: {
schoolId: ObjectId("5d91c9ec098506001b426cb5")
}
}, {
$lookup: {
from: 'students',
localField: 'studentId',
foreignField: '_id',
as: 'student'
}
}, {
$unwind: "$student"
}, {
$match: {
"student.gender": 1
}
},{
$count: 'count'
}])
But it looks more than 10 seconds. I have already add indexes on every ID: exams._id, students._id, exams.studentId, exams.schoolId, student.gender, etc...
Can someone gives me some suggestions in order to make the query faster?
Explains:
{
stages: [
{
$cursor: {
query: {
schoolId: ObjectId('5d91c9ec098506001b426cb5')
},
fields: {
_id: 1
},
queryPlanner: {
plannerVersion: 1,
namespace: 'happya.exams',
indexFilterSet: false,
parsedQuery: {
schoolId: {
$eq: ObjectId('5d91c9ec098506001b426cb5')
}
},
queryHash: '9533F340',
planCacheKey: 'CE7F9610',
winningPlan: {
stage: 'FETCH',
inputStage: {
stage: 'IXSCAN',
keyPattern: {
schoolId: 1
},
indexName: 'schoolId_1',
isMultiKey: false,
multiKeyPaths: {
schoolId: []
},
isUnique: false,
isSparse: false,
isPartial: false,
indexVersion: 2,
direction: 'forward',
indexBounds: {
schoolId: [
"[ObjectId('5d91c9ec098506001b426cb5'), ObjectId('5d91c9ec098506001b426cb5')]"
]
}
}
},
rejectedPlans: [
{
stage: 'FETCH',
inputStage: {
stage: 'IXSCAN',
keyPattern: {
schoolId: 1,
referenceNo: 1
},
indexName: 'schoolId_1_referenceNo_1',
isMultiKey: false,
multiKeyPaths: {
schoolId: [],
referenceNo: []
},
isUnique: true,
isSparse: false,
isPartial: false,
indexVersion: 2,
direction: 'forward',
indexBounds: {
schoolId: [
"[ObjectId('5d91c9ec098506001b426cb5'), ObjectId('5d91c9ec098506001b426cb5')]"
],
referenceNo: ['[MinKey, MaxKey]']
}
}
}
]
}
}
},
{
$lookup: {
from: 'students',
as: 'student',
localField: 'studentId',
foreignField: '_id',
unwinding: {
preserveNullAndEmptyArrays: false
},
matching: {
gender: {
$eq: 1
}
}
}
},
{
$group: {
_id: {
$const: null
},
count: {
$sum: {
$const: 1
}
}
}
},
{
$project: {
_id: false,
count: true
}
}
],
serverInfo: {
host: 'a98010d6dcf4',
port: 27017,
version: '4.2.6',
gitVersion: '20364840b8f1af16917e4c23c1b5f5efd8b352f8'
},
ok: 1,
$clusterTime: {
clusterTime: Timestamp(1597720010, 1),
signature: {
hash: BinData(0, '1PiNaAzDzNRrnZl/mpVJP4oneyU='),
keyId: NumberLong('6819213090182135813')
}
},
operationTime: Timestamp(1597720010, 1)
};