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.