Count the number of revolutions for 15 seconds and multiply by four, that gives you RPM. With practice, in a place with no other traffic, this is not difficult.

The trike with the dog in it, in my sig, and avitar, is 100 lbs loaded. I also have many other types of bikes to be able to compare to the trike.
100 lbs makes everyone slow. That's life. And I have been riding for decades. For me I don't care about being the slowest thing around. I know it's the heavy load. When I ride most of my other bikes the difference in speed is obvious. I also enjoy having the dog along. You get the kid along, enjoy the kid. It does get easier as ride more.