The common bus (essentially like a road for data) used by the program memory and data memory creates what is called the von Neumann bottleneck, which is the limit of the data transfer rate between the CPU and memory (think of it like a 100-km road between destinations). Because the program and data cannot be accessed at the same time, the data transfer rate is smaller than the rate the CPU can work, limiting the effective processing speed of the CPU. It is made to wait for the data it needs to be moved to or from memory.