The problem was traditionally the link between the northbridge (which connects connects the CPU and RAM) and the southbridge (handles peripheral connections like SATA, PCI, PCI-E, etc) components of the motherboard. Intel has now changed their architecture in the last 4 or 5 years to be a single Platform Controller Hub where the CPU provides the functionality previously found in the northbridge chip.
In this particular case, a "faster motherboard" might have helped with pushing updates to the GPU faster, though my guess is that graphics memory becomes the concern with double the resolution (which in turn needs to have updates pushed by the CPU, so it could help anyway)
In modern Intel processors (especially in portable devices), the GPU is integrated on the CPU die, so pushing updates doesn't count (unless you count pulling data from main memory to provide to the GPU).
