Zero impact for most people. Typical laptop apps simply dont use that much memory bandwidth, so all you get is more idle time. In fact the major reason for a higher bus speed is the graphics card -- which are not available even on the high end laptops (until you get to the USD 5K+ workstations).

I once configured a laptop with single channel (instead of dual channel), cutting the memory bandwidth in half (so same as cutting the bus speed in half). Knew this would hurt performance, so benchmarked it extensively. Net was 1% drop in memory intensive workload (and 0% for others). So it is check-box marketing and not a useful feature for most laptops. But those bus numbers sure look impressive.