Let me tell you what I know about how magic works and manifests, and perhaps some answers might appear. First of all the action of magical manifestation actually takes place within the astral dimen- sion. Time and space as we understand it, does not exist there. The effects are as though it had infinite range. As to speed of travel, I would say that it is accurate to say that magic travels at the speed of thought, whatever that might be.
Magical energy is affected by a variety of external forces.
Magical force fields, such as protective pentacles, can slow or deflect the force. Greater forces such as those exerted by deity or high level spirits can certainly alter the course of one's magic. I don't know the actual speed of thought, but it is the speed at which electrical impulses travel across one's neurons, and is considerably less than the speed of light. I don't mean that literally, but as an analogy for the astral. It may be entirely old hat.