You could theoretically have drivers so outdated that it's causing a problem.
So the graph is telling your your GPU is not rendering the scene at 90fps. To turn on motion smoothing, you'll need to update your Nvidia drivers to a recent one past 416.16 and then ensure motion smoothing is enabled - you'll see a noticeable impact in the framerate. Here is a link to the latest driver as of this post, advanced users recommend using DDU to uninstall the old driver cleanly.
What's confusing is why a 1070 would be struggling so hard to render simple games. You still should look at your task manager and see if there is anything else drawing GPU perf. I would also recommend setting your laptop's power mode to "high performance" in the window's power setting and ensuring that the laptop is plugged into the wall whenever you have a SteamVR session open - most laptops will dramatically cut power to the GPU if the laptop is unplugged.
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