Why do programs run more slowly when a computer does not have enough RAM?

Programs run more slowly when a computer has insufficient RAM because the system must rely on virtual memory to compensate for the lack of space.

While RAM acts as a high-speed working area that allows the central processing unit (CPU) to access data rapidly, permanent storage devices like hard disk drives or solid-state drives are significantly slower. When physical RAM is full, the operating system uses virtual memory to store excess code and data on these slower storage devices, automatically swapping blocks of information in and out of main memory as needed.12

Because the rate of data transfer between the CPU and permanent storage is much slower than the speed at which the CPU operates, this constant reliance on secondary memory creates a bottleneck that prevents the CPU from processing instructions efficiently, leading to reduced responsiveness and slower application performance.1