That woud suprise me as wp doesn't have a reputation for being slow.
However for advanced functionality and flexibility I'd go for Drupal as it has a good development framework. However it has a steep learning curve and it takes a while before you figure out how to use it. Once you do get to know it's amazing what you can do with it and how quickly too. It get's a bad rep some times as there are so many idiots out there who just haven't figured out how to use it.