I keep running into this hurdle. I can understand that it would take 'infinite energy' to accelerate a particle to light speed using fields sharing the same time frame of the point of origin of said particle... but I also don't see how this could ever happen. Yet this seems to be the base of most relativity spacetime calculations... along with 'c is constant in every frame of reference'. I don't see how that is possible, unless there's a new 'aether' (a 'speed-limiting substance' hypothesis disproven between 1881~1887)... yet it seems that they're trying to make the boson seem this. And this is, in turn, a base for a 'time dilation at high velocities' theory (hypothesis? I've never seen record of any non-mathematical demonstration of this - I do not consider mathematical theory 'theory' in the scientific sense of the term). In short, this reasoning raises more questions than it 'answers', for me.
If two particles travelling in opposite directions at 99.9~% c collide, their relative velocity would be ~199.87% c. Even before their collision, it doesn't matter if one or both particles are moving: their velocity relative to each other is this.
It's around here that I'm accused of 'thinking Newtonian' and being referred to 'sacred' relativity (and its 'nothing can travel faster than c in any frame of reference' math-apologetics)... but quantum mechanics seems to be just fine with super-c particles.