Massive framerate drops because of rendering?

So It was running 60 fps, but without warning it started decreasing. It´s not constant: sometimes is around 30, 15 and it even fixes itself and starts runing at 60 again. I know that it´s not because of any script or something I made because I tried it in an empty scene, and also in other project, and it was still this low. So it must be something weird on either Unity or my computer.

*Windows 10 64 bits
*Intel Core i5 7300 HQ @ 2.50 GHz
*Intel HD Graphics 630
*Nvidia GeForceGTX 1050

I tried restarting my computer, but it didn´t work. I disabled the in-game overlay in the GeForce Experience software and nothing.

Edit: Apparently, the rendering is expensive… a lot.

when the framerate was good, the rendering was low and instead the VSync was high. Also, the CPU and GPU usage were like at least the double in the task manager when the game was running at 60 fps. So why are these working less?

Does it happen only in play mode? If yes you can test if the problem is Unity, by opening up the profiler (Window → Analysis → Profiler) and clicking at the peaks.

Otherwise, what I do when encountering mysterious framerate drops is opening the task manager (Control+Shift+Escape) which is helpful most of the time, as it shows you the RAM and CPU usage of every process.