a german pc magazine also reports usb 2.0 speeds for iphone 8: "Das iPhone 8 kommt mit dem USB-Standard 2.0, das macht Dateiübertragungen auf den Mac oder Computer etwas zäh (
http://www.computerbild.de/artikel/cb-Tests-Handy-Apple-iPhone-8-Test-15182739.html)." translated: "the iphone 8 comes with usb 2.0. this makes file transfers onto the mac or pc kind of slow." but i also don't trust "computer bild". and they even don't mention the plus that's purportedly usb 3.0 according to gsmarena.
edit: chatting with apple support: the guy verified with a technician that the iphone 8 supports just usb 2.0. awaiting his answer for iphone 8 plus.
edit 2: even after pressing for an answer, he could not confirm usb 3.0 for the iphone 8 plus.
in my opinion that's real letdown. i have a big itunes library and usb 2.0 is the only real thing i hate about my current but old iphone 5.