Imke Feldmann has the need for speed:
In this article I’m going to present a method about transforming multiple columns at once in a fast way. This method also allows you to reference columns that exist in your table already. As I have described in a previous article, this cannot be done using the native Table.TransformColumns function that will be applied if you do column transformations using the UI in Power Query. The function I am sharing here allows you to enter a list of column names to be transformed and a function that defines the transformation itself. So you have to be familiar with defining custom functions to use this approach.
Click through for Imke’s function and explanation but also check out the comments for another take on the problem.