[userbinator]

you can see how long it can take to calculate the size of a folder

On older Windows (95, 98, XP) I've never had it take an unacceptably long time, and that was with a regular HDD. With an SSD and the large file caches which are possible with today's machines with lots of RAM, it should be even faster.

Besides, it's not as if the operation needs to be synchronous; the sizes can be calculated and displayed when they're ready. Having an option to show them would be useful, and those who don't need it/don't like the extra disk activity could leave it off.

[Bjartr]

Older versions of Windows used the FAT filesystem rather than the current NTFS filesystem. FAT happened to get you directory size "for free" due to how it works under the hood, whereas NTFS does not (but it's faster at other things as a result). For much more detail check out this[1] post on The Old New Thing blog by Raymond Chen.

[1] http://blogs.msdn.com/b/oldnewthing/archive/2011/12/26/10251...

[Sharlin]

It is absolutely hideously slow on NTFS to compute the size of a directory that (recursively or not) contains thousands of files. Indeed, it is absolutely hideously slow on NTFS to do just about anything that needs to touch thousands of files at a time.