The tires provide traction, trail clearance, the ability roll over some roughness, and trail/road vibrations. It is the first thing to consider when deciding on roller skis.
The second thing is the ability to control speed. The tires play a role in your speed when roller skiing, but there are a few other factors.
Regardless of skis, one can control speed by selecting appropriate terrain for your comfort level and ability. Staying on flat trails, like many of today’s rail-trails, will allow you to manage your speed to a certain extent. Flat trails and parking lots are great locations for the learning to roller ski, whether you have done some xc skiing or not. Skiing uphill will allow you to increase your workload, at rates that you feel comfortable with. Skis can always be taken off for hills to steep for one to descend.
Another way to control speed is to drag a trailer tire or other object to provide additional resistance. This resistance may allow you to go down a hill at a comfortable pace or will enable you to work at a more intense level at a reduced speed.
There are mechanical speed reducers that can be attached to the ski, providing friction to the rotating wheel. One can easily adjust them before going down a hill or to offer more resistance for a workout on the flats.
Calf brakes can also be used to control along with allowing you to stop quickly. The calf brake is the speed control/braking method that
I prefer. It is available as an add on to some roller skis, mounted on a ski boot with a small bracket, or part of the ski frame on type of roller ski sometimes called a cross skate. I like it so much, and I wish more roller ski manufactures would incorporate it in their standard roller ski package. The video below shows the calf brake on a pair of Skikes being used to stop quickly. The blog post, Canton Ave. Brake Test demonstrates the ability of using a calf brake to control descents. Canton Ave. is one of the steepest streets in the world.
In summary, the ability to control speed is as important as having good traction on the surface your roller skiing on. Choice of terrain and mechanical devices like speed reducers and calf brakes are great for maintaining safe speeds and controlled stops.