Bromine and air take about 15 minutes to diffuse completely but bromine diffuses into a vacuum very rapidly. Why is this so?

Bromine diffuses slowly in the air because the motion of bromine particles is obstructed due to the collision with the moving air particles. But bromine diffuses very rapidly in a vacuum because in a vacuum there is no gas, and the motion of the particles of bromine gas is not obstructed by any other particle. That is why Bromine takes 15 minutes to diffuse in the air completely but bromine diffuses into a vacuum very rapidly.