I can't find any technical information about it, just some Reddit replies from supposedly MS Windows Graphics developer... Some say it pretty much forces Borderless Windowed mode, but true Windowed Borderless mode uses Windows Sync (V-Sync-alike but based on Windows composition) with Triple buffering and lower input lag than with proper Fullscreen + double buffering, but not as low as in Fullscreen mode + no V-Sync. This Optimized Fullscreen mode does not use Windows Sync + triple buffering, yet it is not true Fullscreen mode and it is supposedly, somehow, supposed to increase performance a little bit? Does anyone know more? I know switching from Windowed Borderless mode to the new Optimized Fullscreen mode takes signifcantly less time than switching from Windowed Borderless mode to true Fullscreen mode in pre-Creator's Update builds. Some also say it uses an overlay meant only for GameDVR/GameMode/BCast/Xbox-stuff. If all that Xbox stuff is gone, then is there any reason to use it? You can go into .EXE properties to disable Fullscreen Optimization, but it doesn't really do anything, aside from preventing ReShade (and possibly ENB) from working...
Thanks, but there is no definite answer there either as to exactly what it is and what it does... The Reddit answers I found were closer to answering my questions, but again - not definitely...