Chris Webb makes things go faster:
A few weeks ago I replied to a question on reddit where someone was experiencing extremely slow performance when importing data from a CSV file using Power Query. The original poster worked out the cause of the problem and the solution themselves: they saw that removing all date columns from their query made their Power Query query much faster and that using the Date.FromText function and specifying the date format solved the problem. While I couldn’t reproduce the extreme slowness that was reported I was able to reproduce a performance difference between the two approaches and Curt Hagenlocher of the Power Query team confirmed that this was expected behaviour.
Read on for the example and explanation.