when i tested it out at my minecraft, the beam of the beacon block spins very very slowly. Therefore, i created another world (tested both in creative and survival) and tested on the beacon again, it turns out the beam spins at the normal speed again. I do not know if the speed of spinning beam is related to the size of my world (because it is about 370 mb) of because that this world is created back in the 1.8 pre-release.