I would regard this as Type: resource usage, instead of performance. Given enough RAM, loading the whole directory at once will likely be faster.

The downsides of os.listdir:
a) You can't get a peek at the files so far, it's all or nothing. I only wanted to know if a directory is empty and I have to read the whole thing just to throw it away (maybe I missed another library function?)

b) Using it in a GUI basically requires you to use threads if you may run into a dir with many files. Especially on a slow filesystem (network). Because you won't regain control until the whole thing is read.

I would like to have an iterator version as well, but I also dislike another function (especially the "x" prefix). How about adding a keyword argument to select iterator behaviour?