Your speed doesn't matter, it's the speed at which your head hits the ground that matters and that can be quite high even if you're standing still before falling
The speed you travel on your bike directly contributes to the speed your head potentially hits something. Also the slower you travel the more time you have to react to or avoid or mitigate a crash.
We could require everyone to wear helmets all the time (for their own safety since someone could just fall over or trip at any time), but that's clearly not going to happen.
I think the more general point to consider here is that helmet laws are a distraction from larger issues surrounding traffic deaths and the way we design our cities and transportation systems.
I’d note that being hit by a car doesn’t crush you typically, it merely transfers a lot of force and you hit the ground very hard. One of my best friends in high school was an unparalleled genius in everything he attempted - piano, chess, sports, math, everything. But he was hit by a car driving and hit his head on the curb. He has lived there rest of his life in the care of his parents with the mental capacity of a 5 year old.
Your speed doesn't matter, it's the speed at which your head hits the ground that matters and that can be quite high even if you're standing still before falling