Note that if you parse the MFT itself you can calculate this info for the entire drive in the time explorer can calculate it for a single folder. I know there is some overhead with the official api of course and it needs to support other filesystems but we really needed a way to iterate directories faster just to gather this kind of info. There are several disk space utilization / treesize apps and file search apps that take advantage of the speed you get by bypassing the API.