Javascript queries takes too long to execute in MongoDB

Question

I have two collection "Sample" and "Test" which is 300mb and 4GB respectively. When I tried to query the two collection in a single query I used Javascript. But it takes very long time to execute. How to increase the speed of the query? Below is the query.

db.test.find({}).forEach(function(x) {
  if (x.a == null) {
    print(x)
  } else if (x.a != null) {
    db.sample.find({}).forEach(function(y) {
      if (y._id == x._id && y.c == true) {
        print(x)
      }
    })
  }
})

Also will this query make use of Single indexe which is created on "c" and "a" fields.