tl;dr List.Contains is slow in Power Query because for each iteration, for example each row in your table you check, it scans the list from the start until it finds a match. Buffering helps so it doesn’t re-evaluate on every iteration, but it still scans each item in the list. A better approach is merging via Table.Join, or best of all (in the right scenario), turn your list into a single record to create a hash map and do near constant-time lookups with Record.Fi eldOrDefault.
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