A SQL-backed Power Query refresh becomes much slower after a transformation is added. What would you investigate?

Instruction: Name a transformation and connector you understand. Do not say a disabled View Native Query option always proves that no folding occurs; diagnostic support varies.

Context: Connects a specific regression to data movement and transformation placement while distinguishing folding support from merely using SQL.

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I'd compare the query before and after that transformation and check whether the source is still doing the expensive work...

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