The "green line" playback cache is in ram, whereas the disk cache is on the disk (obviously!)
. So the frames from the disk cache have to be loaded into ram before they can be played. The cache files might also be quite big - such as 32 bit raw RGBA (or more) channels. So they are not likely to perform quite as well as the playback cache. However, they are permanent, at least until they get invalidated by a change.
You can check to see if the disk cache is working by right clicking on a node's disk cache icon in its control area and getting fusion to open the directory where the cache files are. Then you can inspect that they exist and how big they are etc. Also when you render, each node flashes a little light while it is being processed. Any nodes upstream of an unchanged disk cache shouldn't flash, because their result is taken from the disk cache and so don't need to be processed.
I have also sometimes seen the green line not appear, but playback is smooth none the less. So perhaps there is a bug where occasionally the green line doesn't show itself.
DC