MongoDB Aggregate Slow Performance When Using Sort - mongodb

Question

I have collection (tvshow episodes) with more than 1,200,000 document,
here is my schema :
var episodeSchema = new Schema({
imdbId: { type : String },
showId: {type : String},
episodeId: { type : String },
episodeIdNumber:{ type : Number },
episodeTitle:{ type : String },
showTitle:{type : String},
seasonNumber:{type : Number},
episodeNumber:{type : Number},
airDate : {type : String},
summary:{type : String}
});
I created Index for episodeTitle episodeIdNumber seasonNumber episodeNumber episodeId and showId
Now i used mongodb aggregate group to get every tvshow episodes
here is the aggregate query i used :
episode.aggregate( [
{ $match : { showId : "scorpion" } },
{$sort:{"episodeNumber":-1}},
{ $group: {
_id: "$seasonNumber", count: { $sum: 1 } ,
episodes : { $push: { episodeId : "$episodeId" , episodeTitle: "$episodeTitle" , episodeNumber: "$episodeNumber" , seasonNumber: "$seasonNumber" , airDate: "$airDate" } }
} }
,
{ $sort : { _id : -1 } }
] )
Now when i am run this query its take more than 2605.907 ms , after some digging i found out why its slow , it was because of using {$sort:{"episodeNumber":-1}} , without using {$sort:{"episodeNumber":-1}} its take around 19.178 ms to run.
As i mentioned above i create an index for episodeNumber field and based on MongoDB Aggregation Pipeline Optimization
i used sort after match so basically everything was ok , and i didn't anything wrong.
So after this i thought something wrong with my indexes , so i removed episodeNumber index and reindexd , but i had same time nothing changed.
At end one time i tried run aggregate group query without episodeNumber indexed and surprisingly it was faster ! its take around 20.118 ms .
I wants know why this happened , isn't indexes to get faster query ?
Update
query explain output :
{
"waitedMS" : NumberLong(0),
"stages" : [
{
"$cursor" : {
"query" : {
"showId" : "scorpion"
},
"sort" : {
"episodeNumber" : -1
},
"fields" : {
"airDate" : 1,
"episodeId" : 1,
"episodeNumber" : 1,
"episodeTitle" : 1,
"seasonNumber" : 1,
"_id" : 0
},
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "test.episodes",
"indexFilterSet" : false,
"parsedQuery" : {
"showId" : {
"$eq" : "scorpion"
}
},
"winningPlan" : {
"stage" : "EOF"
},
"rejectedPlans" : [ ]
}
}
},
{
"$group" : {
"_id" : "$seasonNumber",
"count" : {
"$sum" : {
"$const" : 1
}
},
"episodes" : {
"$push" : {
"episodeId" : "$episodeId",
"episodeTitle" : "$episodeTitle",
"episodeNumber" : "$episodeNumber",
"seasonNumber" : "$seasonNumber",
"airDate" : "$airDate"
}
}
}
},
{
"$sort" : {
"sortKey" : {
"_id" : -1
}
}
}
],
"ok" : 1
}