I don't have such problem with FFXIV on W10 x64. The only issue I have is the game never minimizes properly when atl+tab which is really annoying. This problem is a FFXIV problem when you use the DX11 client and not a W10 related. Other than that I never got any problem on W10. Be sure to make a clean install by formatting your HDD/SSD AFTER you upgrade to W10 and check online for your latest motherboard drivers. Also note that some AV programs create problems on W10 and FFXIV to some ppl.