In newer version of Jenkins the runs were stored as a timestamp value. Starting with 1.597 the runs are now stored as numbers instead. There is a *one time startup *performance issue when upgrading as this conversion needs to happen. In the Jenkins logs (usually in /var/log/jenkins), there are warnings which indicate the migration is happening. If this continues to happen, then please file a new support ticket.
For more information please go to the open source Jenkins wiki: 1.569+ Migration.