Strange how some people have this issue and others not. Maybe it's region dependent? I have a UK P20 Pro (Three network - dual sim) and I have been using it for navigation for a few days now with no issues. It is pretty much running the same as it did on my Galaxy S8+.
Not noticed lag anywhere so far! Scrolling is better than it was on any of my previous Samsung devices.
My only issue is the stock icons and launcher. It looks too outdated. I'll stick Nova on it soon when I have the time to set it all up.