First it gets the id's of all followers.
Now instead of doing this...
Select.......
FROM....
WHERE.... IN (ids)
I bet it's doing this.
Select.....
From....
Where.... = id
This means that it will run a query per each follow ID.
I think it uses the same query used for fetching profile feeds where you only need to match the user ID.
So it simply loops though the followers id's utilizing this select one ID query