improve drupal 7 multisite performance
Drupal
When it comes to multiple sites, you should consider what would be effects of managing multiple sites. Having a separate MySQL database for each Drupal site would means that your could do the following without disturbing others

take one Drupal Site offline
load new content into one site
alter contents of one site

Consequences

Upgrading Drupal content for one site may possibly affect others that are still running previous versions
Doing an Upgrade requires a full shutdown of all sites
Features for improving performance of one site (such as Boost Cache) may hurt performance for other sites

MySQL
When it comes to MySQL, you will have to consider one of two things

Using MyISAM Storage Engine
Using InnoDB Storage Engine

MyISAM and InnoDB have strengths and weaknesses.

MyISAM only caches Index Pages
InnoDB caches data and index pages
InnoDB can also be configured to have InnoDB bypass OS caching
InnoDB can be configured to access multiple cores/CPUs
There are some instances where MyISAM can be as fast or faster than InnoDB
As of MySQL 5.5, only MyISAM supports FULLTEXT indexing. MySQL 5.6 will introduce FULLTEXT indexing to InnoDB

Please see my past posts about other Pros and Cons:

May 03, 2012 : Which is faster, InnoDB or MyISAM?
Sep 20, 2011 : Best Of MyISAM and InnoDB
Jun 07, 2011 : How do I convert a database from MyISAM to InnoDB?
Apr 15, 2011 : https://drupal.stackexchange.com/questions/1715/what-would-be-the-optimal-mysql-configuration-for-a-drupal-7-site/2367#2367
Apr 14, 2011 : What are the main differences between InnoDB and MyISAM?