I can speak only for myself, but generally, the straighter up you are, the slower you'll tend to go. For a few reasons: you're not as aerodynamic when sitting up tall (and headwinds make this super apparent) and that position just isn't as efficient for pedaling.

That part generally isn't subjective, but comfort (for distance) really is. Some riders are more comfortable with a more forward riding style. This generally puts more weight on the upper body and less on the saddle. Some riders are more comfortable with a more upright riding style. This generally puts less weight on the upper body and more on the saddle.

You'll find, if you shop for saddles, that they're often designed with the riding position in mind. A super thin or firm saddle can work well for forward riding positions because the saddle carries less weight, but this type of saddle is not as good for upright positions because more weight is carried by the saddle. Saddles intended for more upright positions are often padded thicker and also are often shaped to provide just a little support in front of the pelvic area. This works sitting upright, but if you lean forward in your riding position, you can really put a lot of pressure on your "soft tissue" with that type of saddle. (Example: the Roll saddle sort of curves up at the front -- okay for sitting upright; not okay for riding forward.)

Ergonomics are a package, and you need equipment that goes together.

Regarding gearing, there's a whole lot more range with a 21/24 speed than with a 7 speed. I presume you asked that because you're considering a base model Roll. It has a 42T chainring with a 14-34T freewheel. The easiest gear on that will be fairly difficult for any moderate incline, especially with its upright riding posture. The Roll Sport, with a triple chainring in the front of 28-38-48, and the same 14-34 freewheel, gives you a much wider range of gears. If you have ANY hills around you at all, I would recommend the model with multiple chainrings.