In MongoDB, is using $in search faster than multiple single searches?

Yes, using $in is faster. Let us see an example and create a collection with documents −

> db.demo653.insertOne({subject:"MySQL"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5ea04b274deddd72997713c0")
}
> db.demo653.insertOne({subject:"MongoDB"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5ea04b304deddd72997713c1")
}
> db.demo653.insertOne({subject:"Java"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5ea04b354deddd72997713c2")
}
> db.demo653.insertOne({subject:"C"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5ea04b384deddd72997713c3")
}
> db.demo653.insertOne({subject:"C++"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5ea04b3b4deddd72997713c4")
}

Display all documents from a collection with the help of find() method −

> db.demo653.find();

This will produce the following output −

{ "_id" : ObjectId("5ea04b274deddd72997713c0"), "subject" : "MySQL" }
{ "_id" : ObjectId("5ea04b304deddd72997713c1"), "subject" : "MongoDB" }
{ "_id" : ObjectId("5ea04b354deddd72997713c2"), "subject" : "Java" }
{ "_id" : ObjectId("5ea04b384deddd72997713c3"), "subject" : "C" }
{ "_id" : ObjectId("5ea04b3b4deddd72997713c4"), "subject" : "C++" }

Following is the query to use $in and search faster than multiple single searches −

> db.demo653.find({subject:{$in:["MySQL","C++","C"]}});

This will produce the following output −

{ "_id" : ObjectId("5ea04b274deddd72997713c0"), "subject" : "MySQL" }
{ "_id" : ObjectId("5ea04b384deddd72997713c3"), "subject" : "C" }
{ "_id" : ObjectId("5ea04b3b4deddd72997713c4"), "subject" : "C++" }