I think no need to share cache and/or cache_public folders, each server can have their own copy. It maybe shared folder isn't working fast enough and making separate copies of cache folders can speed up your site.

There are many optimizations steps which can be implemented, such as

• DB server optimization and maybe separating or maybe istead moving it wo th the web-server
• PHP optimization (configure php fpm pool, enable and configure zend cache)
• enable remote storage
• enablich caching on different levels
• adding different metrics to fine tune the settings

But it's more custom work than some general guidlines for every website