Hi Barry.
It is weird that you are experiencing no peculiarities. With the latest and greatest 10.5.3, I was experiencing very bad orbiting on a model with 42,000 faces.
If I edited it as a component with 5 or 6 copies, it would lag terribly. If I selected a large portion of the model, I also got very laggy performance.
Yes, the orbit tool has always worked with no problems for me, too; I am referring to orbiting by holding down the middle mouse button.
I did not double click anything, it is just that the orbiting performance seemed to improve if I set the double click delay to longer.
Truth be told, I was having worse performance today, with the latest OS than I was with 5.2.
Thanks!