Farmall Cub

The IH and Woods mower deck blades are quite a bit thicker (heavier) than a modern mower.
The spindles also turn less RPM's than a modern mower. Higher blade RPM's allow for greater groundspeed.
My 184 does just fine at lawn mowing in high range second gear. (3.8mph) It doesn't really outrun the mower.

Also, look at the profile of the blades and the shrouding around the blades. The shrouding provides better lift (sucks the grass upward before cutting it off) and cuts the grass into smaller pieces before expelling it from the deck. This leaves less clippings laying on top of the lawn. The smaller pieces fall into the grass.

The modern decks also have a roller or a drag on the rear edge of the mower deck to lay the grass back down in the direction you are mowing. This gives the striping look to a lawn. (A play from the Simplicity book).

I haven't been disappointed since selling my Simplicity. The 184 is a great upgrade and provides miles of during seat time.

This thread is moving away from my original question, but to add some further thoughts....

Early (before say the 1980s or so) rotary mower decks often had simple construction, with just a flat panel to which the blade mandrels were mounted. The underside was often just open space without much in the way of baffling or ductwork. The early Woods decks look like this. More modern mower decks (as on any recent lawn tractor or zero turn) have deeply sculpted decks, with baffles surrounding the blades on the underside, to direct the airflow. Many modern decks also have openings around the mandrels, so that air can be pulled in from the top of the deck. The point of all this is to increase the airflow to the underside, to better blow clippings out the side. Deck designers now use computer simulations and (before that) wind tunnel studies to optimize the airflow.

In my "other life" I have 3 John Deere garden tractors from the 70s, 80s and 90s. The newer mower decks give consistently better cuts that the older ones, and clog less often.

Not that you can't cut grass with an older deck, but just that the newer ones do a better job.