Do you have any programs like EVGA Precision X or MSI Afterburner installed on your computer at all? Programs such as these have been found in the past to be the culprit of this capped FPS issue, and sometimes people forget about disabling them.Sorry for a late response, I got busy over the past couple of days.
I do not believe I have such programs installed. The only item that may fall under the same category is NVIDIA GeForce Experience. Yet, I do not believe that it applies to osu! at all, as it does not recognize it as an official game... I simply use GeForce Experience to update my NVIDIA drivers, which are indeed of the latest build.
Interestingly enough, this seems to be a widespread issue. I have realized that osu! is set to run on integrated graphics by default, as seen on both of my brothers' laptops. In terms of specifications, their computers are of greater quality than mine.
On both computers, I have attempted to run osu! via their NVIDIA GPUs - NVIDIA GTX 1050 and 960mx. Being in fullscreen, the same issue occurs! A fluctuating 60 FPS cap, with 17ms of input lag. This is not the case when you run it in integrated graphics, nor in windowed w/ a NVIDIA GPU.
Could this be some kind of issue regarding NVIDIA GPUs + fullscreen in osu!...?
Any feedback would be appreciated. I apologize for any inconvenience in the delayed reply!