We can already activate debug mode, review the queries and compare what happens when you turn an addon on or off. But asides from this its not easy to find problematic addons. Especially when you run a lot of addons (which many people do) and when your traffic fluctuates from minute to minute. If php files are included in a page then its very hard to identify if one of those files is causing issues. For example: my forum index is loading 118 php files. It would be very beneficial if XenForo would automatically identify addons, files, queries which are loading slow and bring this under the attention of the admin.