Sadly the FBA emulator (along with a lot of desktop applications) has issues with input lag because of the forced Desktop Window Manager in Windows 8.1 onwards. If DWM doesn’t ring a bell, think Aero from Windows 7. It’s basically the same thing. DWM is now so tightly integrated into the OS that there is no option to disable it without resorting to some ugly hacks. With DWM completely disabled (via hack), there’s noticeably less input lag and tearing compared to running the latest FBA with the “Win7 DWM fix”.
I’m actually surprised that not many people realise that Windows 8 (and therefore 10) all have a higher in-built lag than Windows XP. This affects ANY application that is running on the desktop and using the standard APIs to draw their UI (i.e. basically every program in existence). One of the workaround hacks I mentioned is to download Process Explorer, run it as Administrator, and then 1) Suspend winlogon.exe, 2) Kill dwm.exe. This actually stops the DWM, with the side effect that you get an ugly black bar on the side of your screen. On the plus side FBA feels a lot more responsive, on par with running on XP.