When you join a table to another table in Power Query, the UI gives you the option to either expand the columns (default) or aggregate the contents of the joint tables. That’s useful if multiple rows are returned for the rows of the table that has been joined to (left table):
But this method is extremely slow. Compared to “simply” expanding all values to new rows (which took around 5 seconds), the aggregation took around 50 seconds. The automatically generated code uses the “Table.AggregateTableColumn”-function.
Read on to see two separate attempts to speed things up.