An ASP.NET Core endpoint gets slower as the page size grows because EF Core runs many small queries. How would you diagnose and fix it?

Diagnose an EF Core N plus one query in an API endpoint

Diagnoses hidden per-row EF Core queries and replaces them with a bounded query shape without creating an oversized join.

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I would reproduce the endpoint with a representative page size and inspect EF Core command logs or dependency traces for that single request. One parent query followed by the same child query once per parent confirms N plus one. I would then shape the response deliberately: project the required DTO in one query, aggregate in SQL, or load related rows in one additional set-based query. The goal is a bounded query count, not simply adding Include everywhere.

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N plus one detection, lazy loading, query count, projections, Includes, split queries, and endpoint guardrails.

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N plus one is a query-shape failure: one request loads a set of rows, then navigation access or loop logic issues another query for each row. Query count grows with result size even when each individual command looks fast.
I would reproduce the endpoint with a representative page size and inspect EF Core command logs or dependency traces for that single request. One parent query followed by the same child query once per parent confirms N plus one. I would then shape the response deliberately: project the required DTO in one query, aggregate in SQL, or load related rows in one additional set-based query. The goal is a bounded query count, not simply adding Include everywhere.
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I would find what triggers the extra commands. Common causes are lazy-loaded navigation access inside a loop, mapping code that touches unloaded relationships, or serialization walking an entity graph after the endpoint appears finished. For scalar summaries I prefer a translated projection; for a real aggregate I compare , split queries, and explicit set-based loading. I inspect generated SQL and row counts because one giant join can remove N plus one while creating cartesian multiplication, duplicate data transfer, and worse memory use.

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Fixing N plus one by enabling Include everywhere.
Ignoring lazy loading triggered during serialization.
Returning huge entity graphs when a DTO projection is enough.
Leaving page size unbounded after the query is fixed.

An ASP.NET Core endpoint gets slower as the page size grows because EF Core runs many small queries. How would you diagnose and fix it?

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