Since Windows 98, we used Idoswin (https://www.idoswin.de/) which always calculated directory sizes in the background. It showed the sizes of directories it already calculated and made the field empty for every folder where the size was not ready yet. Usually the small directories are ready first and only the big (>1gb and many files) take a few seconds. This is done in the background and therefore doesn't disrupt your workflow at all.
Another method would be to make it midnight-commander style: add a button which will calculate & display the sizes of the directories.
So the time to calculate it is not really an issue.
Use-cases are exactly cleaning up space and roughly calculating backup sizes/sizes of folders I want to copy at a single glance. You could also ask: why do we need to display the size of files in the explorer. The reasons are roughly the same.
Another method would be to make it midnight-commander style: add a button which will calculate & display the sizes of the directories.
So the time to calculate it is not really an issue. Use-cases are exactly cleaning up space and roughly calculating backup sizes/sizes of folders I want to copy at a single glance. You could also ask: why do we need to display the size of files in the explorer. The reasons are roughly the same.