How is DaisyDisk so fast?
I'm the developer of DaisyDisk. I'd have to go into lengths to explain how we achieve this on the engineering side, but I can reassure you that the app doesn't make any "shortcuts" or "tricks". The scanning is real and full. 

As already mentioned, it's kind of hard to make a precise measurement because of the disk caching. Every experiment will give you a different time, depending on many factors. But it's true that DaisyDisk is by far cry faster than any other disk scanner. This is especially noticeable on SSD drives. Haven't tried to compare it with "du" though.