I think that rasterization is easier to parallelize than ray tracing, since raster engines essentially multiply each vertex with the same matrix, while for ray tracing you have to traverse an octtree for each step in each ray. ( Both are of course easy to parallelize, I just think that in general you will run into memory bandwidth problems faster with ray tracing algos.)
Yes, memory bandwidth is one of the biggest slow downs of ray tracing. Rasterization access memory very linearly which makes it very easy to cache and easy to optimize in hardware on the GPUs. But ray tracing access memory very randomly, two rays originating from pixels next to each other easily diverges and so hits objects far from each other in the scene. This means that ray tracing will spend most of its time waiting for memory.
