ways to optimize the website, need criticism

Given: a not very big portal, 7 servers serving it in two data centers, the new approach to its telematic support has almost been fully tested, we would like to hear criticism.

so…
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General:
The site is written in php5, it uses mysql as a database, apache2 as a backend, nginx as a frontend, memcache as a cache, in some cases smarty caching is used (a static copy is completely created for unregistered users and search engines), the eaccelerator for php5 is used, the template engine - smarty


DB:
master-slave replication is used, both data centers are used by one master, there are slaves in each DC, if necessary, an additional slave is installed

Files:
One file repository is used in which all static files are written, return comes from cache servers (nginx via proxy_store (interception 404 and subsequent download) from storage, files on cache servers are killed at atime, the portal core generates unique names for all static files, so that proxy_store works correctly) there can be a lot of cache servers, load balancing is based on views & round robin bind, the benefit of the approach is that you can install cheap servers, you do not need a raid for server caches, you can speed up by smearing files drives.

Core:
The kernel is able to handle master loss (limited functionality) and slave errors (everything is bad), can use cache requests in memcache for registered users, and smarty full-page caching for search engines.

Which way to dig more for optimization?