Having trouble with a slow MongoDB aggregation query - performance

Question

I have the following aggregation query in MongoDB
return mongoose.model('Submission')
.aggregate([
{ $match: { client: { $in: clientIds }, admin: this._admin._id } },
{ $sort: { client: 1, submitted: -1 } },
{ $group: {
_id: '$client',
lastSubmitted: { $first: '$submitted' },
timezone: { $first: '$timezone' },
} },
])
.exec();
which is performing really badly on a collection with about 2000 documents. It usually takes 5 seconds to complete and I've seen as bad as 15 seconds. I have the following index on the submissions collection:
{
client : 1,
admin : 1,
assessment : 1,
submitted : -1,
}
I'm stuck as to why it's taking so long. Any suggestions?
EDIT
I've run the query
db.submissions.aggregate([
{$match: {
client: {$in: ['54a4cdfdd0666c243035dc98','55cc985291a0ffab6849de34']},
admin: '542b4af8880fc300007eb411'
}},
{$sort: {client:1, submitted: -1}},
{$group: {
_id: '$client',
lastSubmitted: {$first: '$submitted'},
timezone: {$first: '$timezone'}
}}
], {explain: true})
in the shell with explain and got
{
"stages" : [
{
"$cursor" : {
"query" : {
"client" : {
"$in" : [
"54a4cdfdd0666c243035dc98",
"55cc985291a0ffab6849de34"
]
},
"admin" : "542b4af8880fc300007eb411"
},
"fields" : {
"client" : 1,
"submitted" : 1,
"timezone" : 1,
"_id" : 0
},
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "webdemo.submissions",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"admin" : {
"$eq" : "542b4af8880fc300007eb411"
}
},
{
"client" : {
"$in" : [
"54a4cdfdd0666c243035dc98",
"55cc985291a0ffab6849de34"
]
}
}
]
},
"winningPlan" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"client" : 1,
"admin" : 1,
"assessment" : 1,
"submitted" : -1
},
"indexName" : "client_1_admin_1_assessment_1_submitted_-1",
"isMultiKey" : false,
"direction" : "forward",
"indexBounds" : {
"client" : [
"[\"54a4cdfdd0666c243035dc98\", \"54a4cdfdd0666c243035dc98\"]",
"[\"55cc985291a0ffab6849de34\", \"55cc985291a0ffab6849de34\"]"
],
"admin" : [
"[\"542b4af8880fc300007eb411\", \"542b4af8880fc300007eb411\"]"
],
"assessment" : [
"[MinKey, MaxKey]"
],
"submitted" : [
"[MaxKey, MinKey]"
]
}
}
},
"rejectedPlans" : [ ]
}
}
},
{
"$sort" : {
"sortKey" : {
"client" : 1,
"submitted" : -1
}
}
},
{
"$group" : {
"_id" : "$client",
"lastSubmitted" : {
"$first" : "$submitted"
},
"timezone" : {
"$first" : "$timezone"
}
}
}
],
"ok" : 1
}
EDIT 2
The output I get from db.submissions.getIndices() is
[
{
"v" : 1,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "webdemo.submissions"
},
{
"v" : 1,
"key" : {
"client" : 1,
"admin" : 1,
"assessment" : 1,
"submitted" : -1
},
"name" : "client_1_admin_1_assessment_1_submitted_-1",
"ns" : "webdemo.submissions",
"background" : true
}
]