It would be helpful to know exactly what code/process is executing this, as noted by  Andrew. You might be performing quite a bit of unnecessary work and overloading the system. Have you checked latency and metrics on your storage? If you're reindexing and updating all statistics, you might be rewriting the entire database. I wouldn't expect this to take 16 hours on 300GB, but depending on your hardware resources, perhaps things are just overloaded. Is this a physical instance or a VM?