I'm experiencing the same issue on my 8+ and it happens regardless of having no apps open or several apps open. I don't think it's a RAM issue, pretty sure it's just iOS 11 being garbage. I also think some people notice it more than others (I'm a pixel peeper and nitpicker). However another much more obvious thing that I see constantly is frame rate drops in general iOS animations (multi task switcher, 3D touch, unlocking screen).