$in slower when using indexed column - mongodb

Question

I am trying to optimise my query and have found that when using $in on a non-indexed column that the performance appears to be faster than when on an indexed column.
For example:
I have added an index on myCollection: {"entryVals.col1" : 1}.
To confirm:
db.myCollection.getIndexes()
returns:
[
{
"v" : 2,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "myDb.myCollection"
},
{
"v" : 2,
"key" : {
"entryVals.col1" : 1
},
"name" : "entryVals.col1_1",
"ns" : "myDb.myCollection"
} ]
I then run a count with a query (printing the time taken) on both the indexed and non-indexed columns.
Count on indexed column
var a = new Date().getTime();
db.myCollection.count({"entryVals.col1": {$in:["a","b","c","d"]}});
new Date().getTime() - a;
returns
96 (time in ms)
Count on non-indexed column
var a = new Date().getTime();
db.myCollection.count({"entryVals.col2": {$in:["a","b","c","d"]}});
new Date().getTime() - a;
returns
60 (time in ms)
Please bare in mind that I ran the queries several times and took an average (there were little to no anomalies) .
Is anyone able to help enlighten me as to why the query on the column that is indexed is slower please?
Thanks in advance.
Explains
Count on indexed column
db.myCollection.explain().count({"entryVals.col1": {$in:["a","b","c","d"]}})
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "myDb.myCollection",
"indexFilterSet" : false,
"parsedQuery" : {
"entryVals.col1" : {
"$in" : [
"a",
"b",
"c",
"d"
]
}
},
"winningPlan" : {
"stage" : "COUNT",
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"entryVals.col1" : 1
},
"indexName" : "entryVals.col1_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"entryVals.col1" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"entryVals.col1" : [
"[\"a\", \"a\"]",
"[\"b\", \"b\"]",
"[\"c\", \"c\"]",
"[\"d\", \"d\"]"
]
}
}
}
},
"rejectedPlans" : [ ]
},
"serverInfo" : {
"host" : "obfuscated",
"port" : obfuscated,
"version" : "3.4.6-1.7",
"gitVersion" : "obfuscated"
},
"ok" : 1
}
Count on non-indexed column
db.myCollection.explain().count({"entryVals.col2": {$in:["a","b","c","d"]}})
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "myDb.myCollection",
"indexFilterSet" : false,
"parsedQuery" : {
"entryVals.col2" : {
"$in" : [
"a",
"b",
"c",
"d"
]
}
},
"winningPlan" : {
"stage" : "COUNT",
"inputStage" : {
"stage" : "COLLSCAN",
"filter" : {
"entryVals.col2" : {
"$in" : [
"a",
"b",
"c",
"d"
]
}
},
"direction" : "forward"
}
},
"rejectedPlans" : [ ]
},
"serverInfo" : {
"host" : "obfuscated",
"port" : obfuscated,
"version" : "3.4.6-1.7",
"gitVersion" : "obfuscated"
},
"ok" : 1
}