Finally, how do I delete 2019? Is it a case of just deleting the files from the Application folder?
Just like removing any other application. There would be a few folders and files in the User/Library/AppData directory similar to those for SU2018. Those may need manual removal.
Thank you 
Hey, maybe Sketchup 2020 will solve the problem
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Hey Michael, Dave-
Some aspects of zooming in LayOut are improved, and some are not.
LayOut now has a preference to control zoom speed - this allows people to customize how quickly LayOut attempts to zoom in & zoom out. People with smaller, more efficient files will find this useful.
However, for larger files, or while editing a group, LayOut can run into performance issues which will result in lagging during zoom. This is a hard limitation that depends somewhat on the specs of the computer, the size of the file, and what’s on the screen during the zoom. In this case, the zoom speed preference will have no effect because the computer is already working as hard as it can.
The LayOut team is definitely aware of this issue and has been working on ways to improve it. For the time being, know that the following things will slow down performance the most: editing inside a group, having vector-rendered viewports on a page, and to a certain extent having large inserted images.
Thanks,
Marc
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Marc has clarified what I meant. My comments are about performance, of which the lag associated with zooming and panning are the issues. The Mouse Scroll Speed has very little implication on this.