Why $group is very slow in aggregation and how can faster it in MongoDB
As per MongoDB Documentation here Groups documents by some specified expression and outputs to the next stage a document for each distinct grouping. The output documents contain an _id field which contains the distinct group by key. The output documents can also contain computed fields that hold the values of some accumulator expression grouped by the $group’s _id field. $group does not order its output documents.

The $group stage has the following prototype form:

{ $group: { _id: &lt;expression&gt;, &lt;field1&gt;: { &lt;accumulator1&gt; : &lt;expression1&gt; }, ... } }


The _id field is mandatory; however, you can specify an _id value of null, or any other constant value, to calculate accumulated values for all the input documents as a whole.

$group Operator and Memory

The $group stage has a limit of 100 megabytes of RAM. By default, if the stage exceeds this limit, $group will produce an error. However, to allow for the handling of large datasets, set the allowDiskUse option to true to enable $group operations to write to temporary files.

As per blog of @VladMihalcea here the  MongoDB aggregation framework is extremely useful and its performances can’t go unnoticed.that didn’t require any extra optimization, aiming to demonstrate the out-of-the-box performance of MongoDB.

For further your ref here