I think the official answer should be rephrased to:
The reason why deletion is so slow in UE4 is that it loads all the assets you are deleting to check for their dependencies across your entire project.
You usually want to rely on that if you are unsure how the content of the folder was used across the project.
But sometimes as it’s your case you know for sure. Then the fastest way is to close the editor and delete the folder manually from your OS.