Assume you fetch a single customer entity with their 100 order entities as includes. With single query this will join both tables and produce 100 rows that contain the order data but also each one contains the customer data redundantly. Now Imagine you had two includes there, that will multiply the number of rows again.

AsSingleQuery is as dangerous as this makes it sound. This works surprisingly well if you know that the number of included entities is low, but only then.

You can get much better queries here if you write a Select() and let EF Core translate that into SQL. That will probably do roughly want you are imagining here, usually with subqueries fetching the data from related entities.

Databases are incredibly smart when it comes to fetching related data, a single select is indeed better than splitting queries and doing multiple roundtrips.

The problem however is in how results are returned over the wire. Duplicating rows is needless, but seems to be still the standard.