Why do SQL Server Scalar-valued functions get slower?

In most cases, it's best to avoid scalar valued functions that reference tables because (as others said) they are basically black boxes that need to be ran once for every row, and cannot be optimized by the query plan engine. Therefore, they tend to scale linearly even if the associated tables have indexes.

You call the function two times (two select hits to the DB) for every row in the result set.

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