Thanks for quick response. Your explanation makes complete sense and certainly clarifies why and when the unique key was dropped. Thank you for the information.
As we are only concerned with the last two weeks rather than hourly or smaller, I was able to merge the existing summary records and add a custom key to the table to allow updates rather than inserts. This will work much better for us and greatly reduces database load (56K vs 4M records and 0.05s vs 7+s for the LEFT JOIN with wp_posts).
With that being said, do you plan to keep the ON DUPLICATE KEY UPDATE part of the view count update in future releases?
Thanks again for the response and the great plugin!