The amount of work required to keep up with a pack is not reduced by using a lower cadence. Power is torque times cadence. It takes a certain amount of power to maintain a given speed. If cadence is reduced, more torque is required and that generally fatigues the legs just as much or more.

Everyone is a little different and some people might find that their legs fatigue less use more torque at 90 rpm than someone else using a lower gear at 100 rpm, but that's not usually the case.

A 50/11 gear at 100 rpm will produce 1mph more speed than a 53/12 and 2mph less than a 53/11. A rider with a 50/11 would have to pedal at 106 rpm to match the speed of a 53/11 at 100 rpm.

A 53/11 at 90 rpm would produce 34 mph. That's a pretty fast pack.