I've actually never seen a single working example where a portcullis dropped 'fast', and the only references to it always reeked of Victorian era 'I've never seen it, but this is logically the way it worked' thinking. They are usually fairly well balanced so they are both easy to raise and lower, meaning you still had to crank them closed.

The other flaw with hinging it at the top is that in order to close the door, you have to step well back from it, or risk getting knocked in the back of the head. That means your defenders have to be further back from the door way, which in turn means their sharp pointy things can't keep the attackers as far away. And an attacker that is closer to the door is one who stands a better chance of keeping it open.
With a side hinge you can have one guy swinging the door closed, and the defenders side stepping out of the way as the door swings shut. Not only that, but they can also stick their sharp pointy things through the door while it is still open, and thrust through the grate. You can't do that with any door falling from the ceiling.