Prefixes for increased projectile speed to Marrow Shards & Bone Nova do nothing for the Bone Nova skill.
In my test I have Betrayal, Flight of the Femurs, and Bone Hail nodes fully filled for the Bone Golem skill.
I have an armor crafted with the prefix and two highly rolled Fractured Huge Immortal Idols providing for 37%, 28% and 26% in increased projectile speed, respectively. Totaling 91% when equipped.
I have tested by having the golem attack a target dummy and by marking the distance the Bone Nova reaches when casting the Rip Blood skill (with the Marrow Drinker node filled) on the golem to produce novas. There is no discernable difference occurring with any of the items equipped in any combination.
However, this prefix visibly works as intended, increasing the speed and distance the Marrow Shards travels, by nearly doubling it when the gear mentioned above are all equipped. It also appears to equally affect the Bone Splinters skill when you have the node enabled in the skill tree.
Now I think it is nice that the prefix inadvertently affects Bone Splinters for Marrow Shards and I think that feature should remain with the prefix, but I do hope you fix the prefix so it will appropriately improve the Bone Novas of golems.