My wife and I did the Rhino Rider excursion this past October (2014), and we both really enjoyed it. It did get more bumpy/choppy the faster you went. There are parts where the group is restricted in how fast you can go, and other parts where you are free to speed up. Our guides told us that we could spread out at the point where we were free to speed up, but for the most part, the group stayed in a line behind the guide in front. You are supposed to stay a certain distance behind the person(s) in front of you, so if you are in the front of the group, you will feel pressure from people behind you to go faster. I would simply suggest that if you prefer to go a little slower, just stay in the back of the group. There are two guides, one in front and one and back, and they did not pressure anyone to speed up.
I would certainly recommend this excursion, it is a lot of fun and gives you great views of the shoreline. If you have any other questions, I'd be happy to answer them. I know when we were researching this excursion we did not find a lot of info on the disboards about it.