The easiest performance boost for any Drupal site is to install Redis.
This is a key-value store that you can use as a cache to drastically cut
down on the number of database calls Drupal is making. These instructions
are for Ubuntu 12.04.

Prerequisites

You need to install compilation tools as we'll be compiling the most recent
version of Redis from source. TCL is also a requirement.

sudo apt-get install build-essentialsudo apt-get install tcl8.5
Download and Compile Redis

First download the most recent stable Redis version. At the time of writing
this is 2.8.6. Extract the source code and compile. Do this as root:

wget http://download.redis.io/releases/redis-2.8.6.tar.gzcs redis-2.8.6make

After compilation it's a good idea to run the tests. You'll fail at this point
if you didn't install TCL.

make test

Then install Redis on your system, again as root:

make install

There is a script provided that will install the Redis service on Ubuntu.

sudo ./utils/install_server.sh
Drupal configuration

First download the Predis PHP library for Redis into your sites Libraries folder.
Predis library can be downloaded here and should
be installed in sites/all/libraries/predis.

Download an install the Redis Drupal module.

Add the connection details to settings.php and the configuration required to
allow Drupal's cache system to use Redis, rather than the database.
Append this to settings.php...

$conf['redis_client_interface'] = 'Predis';$conf['redis_client_host']      = '127.0.0.1';$conf['lock_inc']               = 'sites/all/modules/contrib/redis/redis.lock.inc';$conf['cache_backends'][]       = 'sites/all/modules/contrib/redis/redis.autoload.inc';$conf['cache_default_class']    = 'Redis_Cache';

Now enjoy super fast Drupal!