Considering that stone arrowheads have been around thousands of years, I guess they would qualify as the first "fixed blade" broadhead.

There really isn't a minimum speed for them. Check your state laws for minimum draw poundage for hunting with a hand-held bow. Keep the blades sharp and with good shot placement they will work fine.

As far as mechanicals, many manufacturers quote a minimum amount of kenetic energy vs. a minimum speed. You could have a 40# bow shooting ultra-light arrows going the same speed as a 70# bow with big heavy hunting arrows. The 70# will have a MUCH higher KE amount.

To open mechanical blades- ittakes work in the physical sense. Work requires energy. Many mech heads have a suggested minimum KE rating of around 55ft-lbs of KE.