Right, that was the huge issue I had with the story. In most respects it seemed to be trying hard to be speculative hard SF in the Arthur C Clarke vein (i.e. fine to introduce exotic new physics, but only very carefully and consistently) so it was very surprising to have that gaping hole at the centre.

The aliens had instantaneous communications, and could directly influence events on earth, but still had to travel at sublight speeds? It wanted at least an acknowledgement of the inconsistency, and a token effort at explanation. As others have noted, it’s not at all clear why they couldn’t simply have killed off the humans remotely.