I found Boston to be a somewhat harder course due to the downhills, something I think can be overcome with correct preparation and execution, but I'd not found it slow.
My understanding is that Boston is a tactical race, run for the win, maybe a CR for bonus money if conditions are right. I can't imagine one would go to Boston with consideration of a world record simply because the course doesn't qualify.
Given that, I'd not attribute past slower times to the Boston course itself, I'd attribute them to the nature of the event. This year, Hall went out hard with the wind and the pack kept step and it finished with two guys putting it all on the line. Were it not for Hall and the wind, there wouldn't be a 2:03.