Thanks for replaying, I have made some optimizations, and right now the DirDefaultExternal is doing good, I can scroll really fast and the images are read very fast, the scroll is really smoth. Just from time to time, have some "jumps" but its really acceptable (tested in a Motorola Defy, which is really old by now, and also tested in a samgung galaxy tab 2, in which there are not "jumps" at all, the scroll is really smooth)
The only problem that I found with DirInternal, is that low-end devices like Samsung Galaxy Y and Y Pro (which are really popular in south-America), runs out of internal memory really fast, they have to be very carefully with the installed apps, and from time to time, need to uninstall some because there are no more storage.
So by now, I'm going for the External Storage, and see what happens