JIRA (And other Atlassian tools) are notoriously bad at scaling. In a previous life, I spent a week pulling a 2,000 user JIRA/Confluence/Bamboo stack off to my then employer's stack -- the site couldn't handle more than 40 concurrent users, and this was when the software was on a beefy box with tomcat and a high-perf MySQL Instance. Saying that Atlassian is enterprise is a stretch -- it might be great for small teams (personally, I'm a fan of request tracker and mediawiki), but, I'd never stick it in the enterprise.