1) on mongo shell paste following to define a js function to output test documents with n fields function TestDoc (n) { var doc={}; doc[ 'lang' ]=[ 'da' , 'de' , 'en' , 'es' , 'fi' , 'fr' , 'hu' , 'it' , 'nl' , 'pt' , 'ro' , 'ru' , 'sv' , 'tr' ][ Math .floor( Math .random() * 13)] for ( var i = 0; i < n; i++) { doc[ 'fld_' +i]= Math .random().toString(34).slice(2)} return doc; } 2) define a function to insert nDocs documents with nFields to a collection function InsertTestDocs (colName, nDocs, nFields) { for ( var i = 0; i < nNocs; i++) { doc=TestDoc(nFields); doc[ '_id' ]=i; db[colName].insert(doc) } } 3) insert 1000000 test documents insertTestDocs( "tmp_col" , 1000000, 100) 4) create index on 'lang' field db.tmp_col.ensureIndex({lang: 1}) 5) run a mapReduce job that simply sums distincts on lang field db.runCommand({ mapreduce: "tmp_col" , map: function () { emit( this .lang, 1); }, reduce: function (key, values) { return Array.sum(values); }, out: { inline: 1 }}) 6) you get results of the following form "timeMillis" : 116705, "counts" : { "input" : 1000000, "emit" : 1000000, "reduce" : 65000, "output" : 13 }, "ok" : 1 7) run same map Reduce except specify a sort at this time db.runCommand({ mapreduce: "tmp_col" , map: function () { emit( this .lang, 1); }, reduce: function (key, values) { return Array.sum(values); }, sort:{lang:1}, out: { inline: 1 }}) 7) you get following results "timeMillis" : 1478708, "counts" : { "input" : 1000000, "emit" : 1000000, "reduce" : 8474, "output" : 13 }, "ok" : 1 Notice that now it takes 1478708 instead of 116705 (that is ~10X slower) when run without sort option