The database server is seldom the source of performance issues on web apps. To make judgments, one would need to review the source code and evaluate both the overall data model and the relative use of in-line SQL (hopefully none) versus stored procedures. With SQL Server, it's also important to make sure that the app avoid using dynamic SQL elements in a manner that would prevent the effective compilation of queries. Personally, I haven't noticed any performance problems with EE. However, the HTML layouts are complex and performance could be negatively affected on PC's with slower processors or insufficient memory. The technology supporting EE is infinitely more complex and more robust than that used for vBulletin.
