It isn't so much a case of the bag material clogging as the reduction in the size of the air path as the bag fills. In most bagged vacuums the bag sits against the sides of the bag chamber. As the bag fills airflow is obstructed. If you can suspend the bag off the sides you don't loose airflow. Look at the expander cages used in several Panasonic canister models and uprights ( MC-CG 885, 887, 983, 985, 917, 937, 955, 957, MC-UG773, 775, 787 with their "Opti-Flow" chamber ) as well as the expander cages used in the Kenmore Intuition and Elite canister models to see what I am talking about.
I have been fooling around with a home made expander cage in an old old old Kenmore from 1982 that very much lost airflow as the bag filled. These have the motor sitting vertically with the fan facing up into the bag chamber underneath a big rectangular "filter" ( 1/16 inch thin open cell foam, basically useless as a filter) housing. The bag as it fills presses against the underside and sides of the lid blocking airflow as dirt takes up the airspace. With the expander cage I have made the bag is suspended away from the sides, top of the bag chamber and off the side of the filter housing. This gives the air a nice unobstructed path from the hose opening, over the top of the bag to the motor. The dirt drops out and falls to the bottom and rear of the bag. Now I fill the bag with no loss of airflow. Same thing by different means with my Metrovac. It is a round steel tube with a suction motor in the back. There is an inner cloth bag the disposable dust bag fits into. The cloth bag acts like an expander cage suspending the bag off the walls of the vacuum and to my great surprise ( considering it is such and old fashioned design ) it too doesn't see to loose airflow as the bag fills.