towards the end of video, there is a cpu clutter caused by the objects count, as cpu spikes cause of to many objects (probably more then 1000 you had initially) and makes you lose fps ... you sure the items you are coded to get destroyed are actually getting destroyed? im thinking you are not destroying the items properly. or not all of them.
Also when i said to many objects continously depending on position or calculations, that also apply to your zombies spawning outside screen trying to get to the car... those add up if you have to many... since they will always try calculate the path to the car... and imagine 100 of them doing that + 1000 objects in screen,... don't know why you have 1000 objects in screen when you could fake that gameplay and quality with 5-10 static objects.
That gameplay done properly just what i seen in the video, should cost around 5% cpu or less on pc around 10% on mobile... optimization, look for stuff you missed out destroying, maybe the road itself? or zombies/bushes?