the tx5 is much faster than triax the reason it ibo's lower is because ibo is based on 30in draw because it only goes to 29 they down grade ibo speeds mine is set on 65 lbs 28 draw with rzs its shooting 225 with nockturnal and 100 gn head

225? Or 325?

The reason the TX-5 is faster is due to it having a 5” brace height vs. a 6” on the Triax.

Not a whole lot of difference in a 5 or 6 inch brace height

1” of brace height makes a lot of difference on a bow.

In the speed, forgiveness or both?

Both, more on speed.

Shaw isn't IBO measured using an arrow that is too light for hunting?

IBO standard is 70#, 30” draw with a 350gr. arrow and no accessories on the string.

How much does a typical hunting arrow weigh?

Average is probably about 425gr.

Archers used not to want a bow with less than 7 inches of brace height and would not think of trying a 5 inch brace height bow. If you can shoot a bow with good form and are consistently shooting good at long yardages. Then you can shoot a 5 inch brace height bow. I still shoot an 80 pound bow on occasion. The technology of the early bows barely broke 220 fps. Newer bows easily break 300 fps. and more. IBO speeds published by manufacturers can be hard to obtain. Its somewhat of a fable unless you are willing to shoot an very light arrow.