You are forgetting her state of mind in all the above. How much effort she puts in will depend on how turned on she is and that affects the overall intensity. As Lucy says, having her hands tied makes her more submissive and for many this makes them feel more erotic and this makes them more likely to give good head.

So I would say it was a combination of you enjoying the look of the thing and her enjoying the feeling of the thing.

Damn... I wish now that I was a Dom so I could enjoy this for myself...