As far as I can tell it's just the top and bottom that are plastic, the central part certainly looks and feels metallic to me.Is the back plastic? I hear conflicting info that states it's just top and bottom and others saying its the whole back central piece.
DId yours come with a clear skin type case this is another thing I heard some people get them others do not?
How you finding the speed I know the Mi A1 had really bad lag after awhile which has not been fixed also had issue with when you touch some areas like text to say select to delete a letter it would register it.
I like Miui myself it's a shame they don't allow you to install it on this phone I know some will say well you can get Redmi 6 which is same phone with Miui but it has no b20 support and is hard to get hold of also it's better to have the option isn't that suppose to be the point of these.
I can't see how anyone ordering this mobile wouldn't get a clear skin type case (I used mine until I purchased another more protective one). It's part of the packaging and in the box, mine was still sealed and I was comforted to see the red "Global Version" sign on the side of the box, like I'd seen in unboxing videos. 4G works a treat with my o2 sim so it has band 20 which my Redmi 3s didn't.
This is a bit contradictory now but despite the fact that I like and bought it for Android One and prompt updates, I have to admit that part of me misses MIUI myself, I personally liked it too; my Redmi 3s was resolutely stuck on April 2017 security patch though and Xiaomi don't look like they are going to update it. I'm happy with what I have chosen though, it's a good, solid mobile, bigger screen, faster processor, more storage, with 4000mah battery and Android One to boot, all for about a 1cm increase in height compared to my Redmi 3s.
The speed is a little hard for me to comment on as it generally seems much faster to me than my old mobile, so I was always going to be impressed with it, I've not seen any lag or touch issues that sound like what happened with the MI A1, I have only had the mobile for 12 days though.
HTH