What is query folding and why does it matter?

Query folding is Power Query translating your applied steps into a single native query that the source database executes, rather than pulling all rows and transforming locally.

It matters for performance: folded steps run on the database, which is usually far faster and moves far less data.

Check it by right-clicking a step and looking for View Native Query. Adding an index column is a common folding breaker, so keep non-foldable steps last.