Since way before Mac OS X came on the scene, we have been able to ask the Finder to Calculate all sizes in order to determine the sum total of readable file size space contained in each folder for the Finder window in question.
I have tested List view sizing of folders on several Macs where it doesn't matter if an SSD is present or not, but Lion is so fast at calculating sizes I am curious if there is some new caching data structure or if Finder is using the metadata information from Spotlight or a similar database to speed up this calculation immensely.
Best Answer
I have not observed Lion to be faster at calculating folder (and package/bundle) sizes during the first time it calculates sizes in a folder. However, subsequent calculations in the same folder do seem to be much faster.
Part of the perceived quickness may be that Finder will immediately show the previously calculated sizes in gray text while it recalculates folder sizes, rather than showing "--" until it's been calculated. After a folder's size has been recalculated, the number will update (if the size has changed) and turn black.
Because the Finder is observably caching previously calculated folder sizes, it's possible that it's only recalculating sizes for folders that have changed since the last time it was calculated.