An order list API uses populate('user') and populate('items.product') and is slow. What is going on, and what would you change?

Short answer

populate() is extra queries (or a buffered join) per relationship, often on every row of the list. For a list, snapshot the fields you need on the order, project tightly, use lean(), and populate only on the detail page — or replace populate with a targeted $lookup after $limit.

Why?

populate() is convenient and easy to overuse. N orders with two populates can become many round trips. The modelling fix is the same as $lookup overuse: if you always need product name and price on the order, they should already be on the line item.

Interview tip

Connect populate() to $lookup. Same join, different API, same cost model.

Common mistake

Adding .populate() in a plugin so every find pays for it.