You do NOT want a GPU to ever reach even close to full usage. Like many, you equate USAGE with being the amount in % that the GPU is USED. It is NOT. USAGE is the amount in % of resources the GPU needs to USE in order to render the maximum amount of frames it can in 1 second.
Picture this, hammer a bunch of nails as fast as you can into the drywall, without missing the nail and hitting the wall. Thats going to take all your skill, and use every single muscle in your hand, wrist, arm to accomplish. What it will not use, is all the strength possible in your hand, wrist, arm as its not needed, its just drywall, just a small hammer. Not a 2 handed sledge hammer. USAGE is the amount of muscle power you apply to the hammering, NOT the amount of muscles you apply, which is 100%.
If the gpu even comes close to 100% USAGE, it has nothing left to give, no more possible fps, no more possible vram use, nothing . So when your dude is running through town and a wall explodes and all those particles go in every direction, the gpu has to deal with all that, and fps bottoms out, because it was already using the maximum resources it could, so must slow down the fps in order to make room for all the new instructions per frame.
ideally, you want usage to be 60-70% at most, just so there IS room for changes, like explosions etc that can have a very large impact on sudden fps. A giant fireball in 4k is massive pixilation, as is a field of grass.