Covered Query Slow on 100 million+ Doc Collection - mongodb

Question

I would greatly appreciate any assistance with the following covered query:
db.searches.find({ $text: { $search: "dragon" }},{product_name:1,_id:0} ).limit(30)
The "searches" collection has 126 million documents and our goal is to achieve sub-second query performance.
The format of the collection is very simple:
{
"_id" : ObjectId("584b15d12bb299260a000006"),
"product_name" : "This is a product name",
"price" : 876
}
Both the index and the System Profile result are below. What we don't understand is why the stages are so slow -- total time is 161 seconds. As well, less common words (e.g. dog) are much faster than common words (e.g. dragon) which seems odd given that we're just asking for 30 documents.
This is running on MongoDB 3.4.0 with Wired Tiger. The indexes should be in-memory since they consume less than 50% of the 128GB of memory on the server.
INDEX
[
{
"v" : 2,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "marketplaces.searches"
},
{
"v" : 2,
"key" : {
"_fts" : "text",
"_ftsx" : 1,
"price" : 1.0
},
"name" : "search_index_name_price",
"ns" : "marketplaces.searches",
"background" : true,
"default_language" : "en",
"language_override" : "en",
"weights" : {
"product_name" : 1
},
"textIndexVersion" : 3
}
]
SYSTEM PROFILE
{
"op" : "query",
"ns" : "marketplaces.searches",
"query" : {
"find" : "searches",
"filter" : {
"$text" : {
"$search" : "dragon"
}
},
"limit" : 30.0,
"singleBatch" : false,
"projection" : {
"product_name" : 1.0,
"_id" : 0.0
}
},
"keysExamined" : 1563505,
"docsExamined" : 1563505,
"cursorExhausted" : true,
"numYield" : 14547,
"locks" : {
"Global" : {
"acquireCount" : {
"r" : NumberLong(29098)
}
},
"Database" : {
"acquireCount" : {
"r" : NumberLong(14549)
}
},
"Collection" : {
"acquireCount" : {
"r" : NumberLong(14549)
}
}
},
"nreturned" : 30,
"responseLength" : 4132,
"protocol" : "op_command",
"millis" : 161315,
"planSummary" : "IXSCAN { _fts: \"text\", _ftsx: 1, price: 1.0 }",
"execStats" : {
"stage" : "LIMIT",
"nReturned" : 30,
"executionTimeMillisEstimate" : 145005,
"works" : 1563538,
"advanced" : 30,
"needTime" : 1563507,
"needYield" : 0,
"saveState" : 14547,
"restoreState" : 14547,
"isEOF" : 1,
"invalidates" : 0,
"limitAmount" : 30,
"inputStage" : {
"stage" : "PROJECTION",
"nReturned" : 30,
"executionTimeMillisEstimate" : 144635,
"works" : 1563537,
"advanced" : 30,
"needTime" : 1563507,
"needYield" : 0,
"saveState" : 14547,
"restoreState" : 14547,
"isEOF" : 0,
"invalidates" : 0,
"transformBy" : {
"product_name" : 1.0,
"_id" : 0.0
},
"inputStage" : {
"stage" : "TEXT",
"nReturned" : 30,
"executionTimeMillisEstimate" : 144258,
"works" : 1563537,
"advanced" : 30,
"needTime" : 1563507,
"needYield" : 0,
"saveState" : 14547,
"restoreState" : 14547,
"isEOF" : 0,
"invalidates" : 0,
"indexPrefix" : {},
"indexName" : "search_index_name_price",
"parsedTextQuery" : {
"terms" : [
"dragon"
],
"negatedTerms" : [],
"phrases" : [],
"negatedPhrases" : []
},
"textIndexVersion" : 3,
"inputStage" : {
"stage" : "TEXT_MATCH",
"nReturned" : 30,
"executionTimeMillisEstimate" : 143819,
"works" : 1563537,
"advanced" : 30,
"needTime" : 1563507,
"needYield" : 0,
"saveState" : 14547,
"restoreState" : 14547,
"isEOF" : 0,
"invalidates" : 0,
"docsRejected" : 0,
"inputStage" : {
"stage" : "TEXT_OR",
"nReturned" : 30,
"executionTimeMillisEstimate" : 143608,
"works" : 1563537,
"advanced" : 30,
"needTime" : 1563507,
"needYield" : 0,
"saveState" : 14547,
"restoreState" : 14547,
"isEOF" : 0,
"invalidates" : 0,
"docsExamined" : 1563505,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 1563505,
"executionTimeMillisEstimate" : 20499,
"works" : 1563506,
"advanced" : 1563505,
"needTime" : 0,
"needYield" : 0,
"saveState" : 14547,
"restoreState" : 14547,
"isEOF" : 1,
"invalidates" : 0,
"keyPattern" : {
"_fts" : "text",
"_ftsx" : 1,
"price" : 1.0
},
"indexName" : "search_index_name_price",
"isMultiKey" : true,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "backward",
"indexBounds" : {},
"keysExamined" : 1563505,
"seeks" : 1,
"dupsTested" : 1563505,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
}
}
}
},
"ts" : ISODate("2017-04-28T14:34:57.320Z"),
"client" : "127.0.0.1",
"appName" : "MongoDB Shell",
"allUsers" : [],
"user" : ""
}
May consider Elasticsearch unless we can tune this further.