Yes, lightspeed <i>is</i> a bottleneck. A particular problem is that the computing elements are presumably vulnerable to radiation damage, thus placing a lower limit on the internal radius of the MB's innermost shell. If MBs are migratory, that's a good reason for expecting them to be associated with cooler, dimmer red dwarf stars -- or to be concentrated some way away from their stellar energy source, leaving a classical Dyson sphere in place to harvest energy and beam it to them via laser or maser.<p>(This stuff got chewed over a lot on the extropians list in the early 90s, when the subject of "Jupiter Brains" -- what you get if you dismantle Jupiter and turn it into computing material -- was first raised. Googling on "neuron star" might provide some transitory amusement ...)