Speed pass from hair

I think I will lose my mind, I can’t count how many threads I’ve created on this subject in the last months - but hopefully this will be the last. So please be patiant with me, one more time…

So, the thing is that I after many attempts have succeded with using vector blur for a render - with one little issue. Namely that my vector blur fails on my hair particles. All I can say about this is that it only occurs when the particles are small - like 0.1 diameter.
A little pic of the issue:

As you see the vector blur doesn’t create solid samples but single strings - as smaller strands the fewer strings.
untitled.blend (565 KB)

My guess is that it’s the speed pass that fail, but I have no idea why :frowning: