Actual in-app performance on the Galaxy S8 is not great. The Google Pixel raised the bar for what smooth Android performance looks like, and the Galaxy S8 just can't keep up. In the above picture you see Android's built-in GPU profiler mode. Each vertical line is the time it takes to draw a single frame, with the horizontal green bar representing 60FPS. Bars under the green line rendered in time, resulting in smooth 60FPS scrolling. Bars that overshoot the line took too long to render, resulting in a stutter in the UI animation.
Even in the lowest resolution possible—1480×720—the Galaxy S8 still drops a ton of frames when scrolling complicated apps like the Play Store. The highly optimized Google Pixel—at its full 2560×1440 resolution—is easily getting frames in on time. And even if things do slow down, there's a sizable amount of render time still.