$lookup is super slow despite the foreign collection being empty

Working with Data
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#1

The base collection has 100,000 documents. All of which will be run through a $lookup stage. The foreign collection that will be looked up is empty.

This query takes 30 seconds.

When I remove the $lookup stage, this query takes 3 seconds.

What explains why it is so slow, despite the foreign collection being empty, having no documents?

Similarly, If I join based _id, which is indexed of course, the $lookup could increase the time of the query by 4 times. I know that the index is used because if I join by some other field that is not indexed, the query actually timeout and returns an error. So how come looking up using an indexed field, _id, increase the query time by 4x?

#2

Can you explain why you are doing a $lookup on an empty collection? What is the purpose of this, a $lookup is for joining two collections but in this case it is empty?

#3

It is empty now. But it will be populated. The foreign collection is to store blocked users. The base collection is the users collection. After filtering the users collection, I further have to remove the blocked users.