before optimization:

mongos> db.testcoll.explain("allPlansExecution").find({a:1}).skip(5000).limit(10)
{
	"queryPlanner" : {
		"mongosPlannerVersion" : 1,
		"winningPlan" : {
			"stage" : "SINGLE_SHARD",
			"shards" : [
				{
					"shardName" : "xxxx_0",
					"connectionString" : "xxxx_0/xxxx",
					"serverInfo" : {
						"host" : "xxxx",
						"port" : xxxx,
						"version" : "xxxx",
						"gitVersion" : "xxxx"
					},
					"plannerVersion" : 1,
					"namespace" : "ycsb.testcoll",
					"indexFilterSet" : false,
					"parsedQuery" : {
						"a" : {
							"$eq" : 1
						}
					},
					"winningPlan" : {
						"stage" : "LIMIT",
						"limitAmount" : 10,
						"inputStage" : {
							"stage" : "SKIP",
							"skipAmount" : 0,
							"inputStage" : {
								"stage" : "SHARDING_FILTER",
								"inputStage" : {
									"stage" : "FETCH",
									"filter" : {
										"a" : {
											"$eq" : 1
										}
									},
									"inputStage" : {
										"stage" : "IXSCAN",
										"keyPattern" : {
											"a" : "hashed"
										},
										"indexName" : "a_hashed",
										"isMultiKey" : false,
										"isUnique" : false,
										"isSparse" : false,
										"isPartial" : false,
										"indexVersion" : 2,
										"direction" : "forward",
										"indexBounds" : {
											"a" : [
												"[5902408780260971510, 5902408780260971510]"
											]
										}
									}
								}
							}
						}
					},
					"rejectedPlans" : [ ]
				}
			]
		}
	},
	"executionStats" : {
		"nReturned" : 10,
		"executionTimeMillis" : 29,
		"totalKeysExamined" : 5010,
		"totalDocsExamined" : 5010,
		"executionStages" : {
			"stage" : "SINGLE_SHARD",
			"nReturned" : 10,
			"executionTimeMillis" : 29,
			"totalKeysExamined" : 5010,
			"totalDocsExamined" : 5010,
			"totalChildMillis" : NumberLong(29),
			"shards" : [
				{
					"shardName" : "xxxx_0",
					"executionSuccess" : true,
					"executionStages" : {
						"stage" : "LIMIT",
						"nReturned" : 10,
						"executionTimeMillisEstimate" : 20,
						"works" : 5011,
						"advanced" : 10,
						"needTime" : 5000,
						"needYield" : 0,
						"saveState" : 39,
						"restoreState" : 39,
						"isEOF" : 1,
						"invalidates" : 0,
						"limitAmount" : 10,
						"inputStage" : {
							"stage" : "SKIP",
							"nReturned" : 10,
							"executionTimeMillisEstimate" : 20,
							"works" : 5010,
							"advanced" : 10,
							"needTime" : 5000,
							"needYield" : 0,
							"saveState" : 39,
							"restoreState" : 39,
							"isEOF" : 0,
							"invalidates" : 0,
							"skipAmount" : 0,
							"inputStage" : {
								"stage" : "SHARDING_FILTER",
								"nReturned" : 5010,
								"executionTimeMillisEstimate" : 20,
								"works" : 5010,
								"advanced" : 5010,
								"needTime" : 0,
								"needYield" : 0,
								"saveState" : 39,
								"restoreState" : 39,
								"isEOF" : 0,
								"invalidates" : 0,
								"chunkSkips" : 0,
								"inputStage" : {
									"stage" : "FETCH",
									"filter" : {
										"a" : {
											"$eq" : 1
										}
									},
									"nReturned" : 5010,
									"executionTimeMillisEstimate" : 20,
									"works" : 5010,
									"advanced" : 5010,
									"needTime" : 0,
									"needYield" : 0,
									"saveState" : 39,
									"restoreState" : 39,
									"isEOF" : 0,
									"invalidates" : 0,
									"docsExamined" : 5010,
									"alreadyHasObj" : 0,
									"inputStage" : {
										"stage" : "IXSCAN",
										"nReturned" : 5010,
										"executionTimeMillisEstimate" : 0,
										"works" : 5010,
										"advanced" : 5010,
										"needTime" : 0,
										"needYield" : 0,
										"saveState" : 39,
										"restoreState" : 39,
										"isEOF" : 0,
										"invalidates" : 0,
										"keyPattern" : {
											"a" : "hashed"
										},
										"indexName" : "a_hashed",
										"isMultiKey" : false,
										"isUnique" : false,
										"isSparse" : false,
										"isPartial" : false,
										"indexVersion" : 2,
										"direction" : "forward",
										"indexBounds" : {
											"a" : [
												"[5902408780260971510, 5902408780260971510]"
											]
										},
										"keysExamined" : 5010,
										"seeks" : 1,
										"dupsTested" : 0,
										"dupsDropped" : 0,
										"seenInvalidated" : 0
									}
								}
							}
						}
					}
				}
			]
		},
		"allPlansExecution" : [
			{
				"shardName" : "xxxx_0",
				"allPlans" : [ ]
			}
		]
	},
	"ok" : 1,
	"operationTime" : Timestamp(1559181796, 1),
	"$clusterTime" : {
		"clusterTime" : Timestamp(1559181798, 1),
		"signature" : {
			"hash" : BinData(0,"1S0T1V2L5FYmW6/bPpcdZCEp4Gk="),
			"keyId" : NumberLong("6684083974416891911")
		}
	}
}