If you're interested in seeing the performance and scalability of MySQL in Zimbra improved then you might like to vote on the RFE: Bug 58747 - RFE - Replace mySQL by Percona Server
Percona Server with XtraDB is an enhanced drop-in replacement for MySQL. It offers performance improvements over MySQL and is fully Open Source.
Have a look at the Feature Comparison and some of the details of the product on the Percona web site. If this is something you'd like to see in Zimbra then vote and add your comments to the RFE - your vote will make a difference.