Why aggregation framework is slower than simple find query - mongodb

Question

I am new to mongodb and came across some strange behaviour of aggregation framework.
I have a collection named 'billingData', this collection has approximately 2M documents.
I am comparing two queries which give me same output but their execution time different.
Query 1:
db.billingData.find().sort({"_id":-1}).skip(100000).limit(50)
Execution Plan:
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "billingDetails.billingData",
"indexFilterSet" : false,
"parsedQuery" : {},
"winningPlan" : {
"stage" : "LIMIT",
"limitAmount" : 50,
"inputStage" : {
"stage" : "SKIP",
"skipAmount" : 100000,
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"_id" : 1
},
"indexName" : "_id_",
"isMultiKey" : false,
"multiKeyPaths" : {
"_id" : []
},
"isUnique" : true,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "backward",
"indexBounds" : {
"_id" : [
"[MaxKey, MinKey]"
]
}
}
}
}
},
"rejectedPlans" : []
},
"serverInfo" : {
"host" : "ip-172-60-62-125",
"port" : 27017,
"version" : "3.6.3",
"gitVersion" : "9586e557d54ef70f9ca4b43c26892cd55257e1a5"
},
"ok" : 1.0
}
Query 2:
db.billingData.aggregate([
{$sort : {"_id":-1}},
{$skip:100000},
{$limit:50}
])
Execution Plan:
{
"stages" : [
{
"$cursor" : {
"query" : {},
"sort" : {
"_id" : -1
},
"limit" : NumberLong(100050),
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "billingDetails.billingData",
"indexFilterSet" : false,
"parsedQuery" : {},
"winningPlan" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"_id" : 1
},
"indexName" : "_id_",
"isMultiKey" : false,
"multiKeyPaths" : {
"_id" : []
},
"isUnique" : true,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "backward",
"indexBounds" : {
"_id" : [
"[MaxKey, MinKey]"
]
}
}
},
"rejectedPlans" : []
}
}
},
{
"$skip" : NumberLong(100000)
}
],
"ok" : 1.0
}
I was expecting same results from aggregation framework and find query but find query returned results in 2sec and aggregation took 16sec.
Although in both the queries, I am sorting my documents in descending order(on the basis of _id) and fetching 50 records after skipping 100,000 records.
Can someone explain me why aggregation framework is working this way?
What can I do to make it performance wise similar to find query?