Damn, I’m a moron. I was mentioning speed, I meant to mention force. Baseball travelling 90mph (at 5 oz) has a kinetic energy (KE) of about 120J on the higher end, whereas an arrow traveling at 66 meters per second (147 MPH) with a weight of 1000 grains (2.28 oz and again, not the heaviest to be had) has a KE of about 140J. I’ve never been hit with a 90mph baseball, but even with a helm on, I’d bet it would stagger me, and during a full gallop, might be enough to have me fall of my horse. Not much will lift me off my horse short of a scorpion bolt or a TNT explosion, but a barrage of super charged baseballs certainly won’t help.
Reference for calculations: http://www.currentmiddleages.org/artsci/docs/Champ_Bane_Archery-Testing.pdf
I adjusted some values, i.e. using a full draw length (~30 in) and 150lb draw strength, which wasn’t the strongest you could find in those years. I also used initial velocity, which would pare down during flight, which is why I tried to use the lower end of values to quickly make up for this.