Inside your osu! folder, right click on the osu!.exe, select Properties, and then Compatibility. Once there, check "Disable fullscreen optimizations."Thanks for the speedy response!
If that does not work, restart you computer, close any programs you have running in the background and disable any overlays you have enabled.
You can also try updating or reinstalling your graphics drivers.
I took a shot at disabling fullscreen optimizations. Following all your directions, I navigated the files to adjust the settings as you indicated. However, it seems that the issue continues to persist: I am still locked at 60-61 FPS, with 17ms of input lag.
Restarting my computer did not do the trick, and I analyze this issue with osu! as the only open program, without any overlays.
I just updated the graphics card via NVIDIA GeForce Experience, hoping that it would work...
Unfortunately, it didn't D:
Please continue to attempt to find a solution. All help is appreciated. If you require any other information, simply ask.