My biggest problem with vBulletin (3.8) is its performance. When you have a lot of posts, especially topics with thousands of replies, MySQL queries can take over a minute and crash the server. Recently I've had to actively seek out longer topics (2k+ replies) and stop them before they cause downtime.
What performance improvements will XenForo have over vBulletin? Is it better optimised for larger forums or are huge topics an inherent problem?