I seem to have fixed the issue but only to a slight extent. Try to clear cache/uninstalling the app completely and then reinstalling. Also try switching on high performance mode. You can also prevent the app from sleeping through device maintenance settings. These steps might help your case but its now starting to get clear that Samsung is purposely slowing down their phones from previous generations. Uptil July my Note8 was running very smoothly but with reduced battery life. Now it's suffering from the performance issue as well.