Ixis utilise the following technologies even on the most simple setups to ensure your Drupal site performs well:
APC ensures we avoid the overhead of parsing and compiling source code on every single request resulting in quicker page execution.
We use Memcached to cache objects in memory reducing reads to a database and allowing for cached data to return quicker.
For high performance we implement the HTTP accelerator Varnish to store data in virtual memory and allowing us to serve a large amount of concurrent requests with a lower footprint than Apache.
Galera, a synchronous multi-master cluster for MySQL