If the shared memory is in main memory, then the CPU cannot update the display faster than is allowed by memory. This can be helped somewhat by getting faster memory. Another problem is the bus itself. If the CPU is contending with other devices on the system bus, then that will reduce the update rate to the monitor. A common solution to this is the local bus or dedicated bus strategy.
Some processors support a dedicated bus or local bus to video memory that bypasses the system bus.
With a local bus, the memory shared between the CPU and video hardware is located directly on the video adapter and the local bus runs from the CPU to a memory controller on the video adapter. PCI is a more general local bus technology that competes with these local video bus standards.