MongoDB very slow result with $lookup - php

Question

I am using PHP to interact with MongoDB, recently I used $lookup on 5 collection and my query starts taking around 20-25 seconds to get the result. All my collections have testing data which is not more than 30-40 entries. I also used profiling but I am getting message: "Query Not Recording (too Large)"
I am sharing code from my PHP file. I have also tried creating index on field which are creating joins between collection but got same result. Please let me the the best way to create collection structure and to retrieve data from MongoDB.
Thanks in advance.
$collection = new MongoCollection($db, 'col_main');
$condition = array();
$condition[] = array(
'$lookup' => array(
"from" => "col_a",
"localField" => "a",
"foreignField" => "_id",
"as" => "col_a"
)
);
$condition[] = array(
'$lookup' => array(
"from" => "col_b",
"localField" => "b",
"foreignField" => "_id",
"as" => "col_b"
)
);
$condition[] = array(
'$lookup' => array(
"from" => "col_c",
"localField" => "c",
"foreignField" => "_id",
"as" => "col_c"
)
);
$condition[] = array(
'$lookup' => array(
"from" => "col_e",
"localField" => "e",
"foreignField" => "_id",
"as" => "col_e"
)
);
$condition[] = array(
'$lookup' => array(
"from" => "col_f",
"localField" => "f",
"foreignField" => "_id",
"as" => "col_f"
)
);
$condition[] = array(
'$project' => $projection
);
$condition[] = array (
'$unwind' => '$col_a'
);
$condition[] = array (
'$unwind' => '$col_a.translation'
);
$condition[] = array (
'$unwind' => '$col_b'
);
$condition[] = array (
'$unwind' => '$col_b.translation'
);
$condition[] = array (
'$unwind' => '$col_c'
);
$condition[] = array (
'$unwind' => '$col_c.translation'
);
$condition[] = array (
'$unwind' => '$col_e'
);
$condition[] = array (
'$unwind' => '$col_e.staff_name'
);
$condition[] = array (
'$unwind' => '$col_e.staff_surname'
);
$condition[] = array (
'$unwind' => '$col_f'
);
$condition[] = array (
'$unwind' => '$col_f.translation'
);
//Some More conditions
$condition[] = array(
'$match' => array( 'col_b.translation.language' => $val )
);
$condition[] = array(
'$match' => array( 'col_a.translation.language' => $val )
);
$condition[] = array(
'$match' => array( 'col_c.translation.language' => $val )
);
$condition[] = array(
'$match' => array( 'col_f.translation.language' => $val )
);
return $collection->aggregate($condition);

Answer 1

As you are multiplying object using many unwind operations this could produce a huge set of data, every $unwind muliplayes documnet set, so there could be a huge memory consumption and even swapping.
if we have 3 arrays 3 elements each, then we will perform unwind on a, b, c
we will get:
3 documents after first step,
9 documents after 2nd step
27 documents after last step..