I have 6 GB RAM and I think not even 3GB was used when I tested it. I find it weird that the lag starts when the FFXIV becomes "hidden". I would have assumed, that considering FFXIV wouldn't have to draw anything out in that mode, that would have been even easier for the computer to handle, but it's not the case. The lag isn't temporary either: as long as FFXIV is minimized or has another window maximized over it, it will cause lag to other programs.