If an actual ad played, I'd be irritated beyond belief. But when there's a 12 second buffer, I have enough patience training for slow load times that I instinctively just quickly check my email or spend a brief moment lost in thought. Especially when it's every video. If it was one in every 5 videos, I'd notice it and be bothered. When it's every video, it's part of the experience and my brain just cuts it out automatically.