Premise[]
A lot of this you're just gonna have to bear with me here for. In D&D it is possible (even extremely easy) to dodge point blank arrows, fired from bows, composite bows, or crossbows. That said, point blank would be a few feet, as each "block" is 5 feet. So... probably 2.5 feet (half a block) if they are right next to each other. Let's check it out.
Calc[]
Normal arrows travel at 225 fps (68.58 m/s), and compound bows can travel at 300 fps (91.44 m/s). Both solidly within Subsonic speeds. Finally, the fastest crossbows go at 440 fps (134.112 m/s).
Now. Any character is able to dodge this. More dextrous characters are more likely to dodge it, but that doesn't mean they will always dodge or that others will always fail.
We know they do not move more than a foot or two, as they never leave their 5 x 5 square, which we will assume they are in the middle of. So. We will assume 1 foot (0.31 meters) as the normal movement.
So. In the time that it took the arrow to move 2.5 feet (0.76 meters), our adventurer can move 1 foot (0.31 meters). We'll use different projectiles as low- to high-ends.
Since we know the speed of the projectile and distance traveled, we will multiply the character's distance by the speed of the projectile, divided by the distance traveled of the projectile. Same as we did here.
Low-End: 0.31 meters moved x 68.58 m/s / 0.76 meters projectile moved = 27.97 m/s, Superhuman
Mid-End: 0.31 meters moved x 91.44 m/s / 0.76 meters projectile moved = 37.31 m/s, Subsonic
High-End: 0.31 meters moved x 134.112 m/s / 0.76 meters projectile moved = 54.70 m/s, Subsonic
Tallies[]
TL:DR, speed upgrade for base level adventurers and the whole verse.
Adventurers Dodge Arrows (Low-End): 27.97 m/s, Superhuman
Adventurers Dodge Arrows (Mid-End): 37.31 m/s, Subsonic
Adventurers Dodge Arrows (High-End): 54.70 m/s, Subsonic