Makes sense, iPhone X has more pixels (2436x1125 = 2,740,500 pixels) than the iPhone 8 (1334x750 = 1,000,500 pixels) or (8 Plus 1920x1080 = 2,073,600 pixels).
If they have the same processors, it makes sense the one pushing more pixels is slower.
Yeah, the phone wasn't being used. Only thing I did before running the test was close all the apps. I ran the test while the phone was sitting on my desk.