What should be the optimum values for Minimum cache lifetime and Expiration of cached pages
The values of minimum cache lifetime and expiration of cached pages depends on how frequently the content on your site changes and for how long the external caches can show stale data from your site.

To get more info about these check https://drupal.stackexchange.com/a/83342/4876

However in your case I am not sure if these will matter. You should probably consider looking at Views Content Cache That might improve the performance in your case.