FPS drop

After recent patch, I am facing some problem about FPS every time I play
The problem is my FPS drops to 20 or below whenever someone throws any kind of grenade except flash at near to my character, or when scorchers in ZM explode. At this instance (3-4 seconds) my screen becomes weary and looks like a rain on a car's windshield. Anyone know what causes this?

On average I get 80-100 FPS on 8v8 .

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