I removed Notion from my Nexus 6P yesterday. As I wrote yesterday, I started to see performance issues with the Nexus 6P shortly after installing Notion. 

As I was using my Nexus 6P it would randomly, suddenly, stop responding, as if the processor is 100% utilized in the background. 

I am not 100% sure the problem was caused by Notion, but it started happening right shortly after installing it. After uninstalling the problem did not go away, so I two additional steps. 

First, I uninstalled the Google Wallpaper app, which I had installed just prior to Notion. Next, I erased the cache partition on the phone. 

Performance seems to have improved, it may be the cache partition was the issue, although when you remove the cache partition that also removes all home screen widgets and I have not replaced all of them, so there may have been an issue with the widgets. 

So, I will continue to monitor, fearing that a full factory reset might be in the future.