Trying to use too many programs at once in a system with too little memory will result in constant disk swapping, called thrashing . Thrashing can cause system performance to slow to a crawl. If you try to run Windows NT on a PC with only 16 MB, the operating system will have to swap almost constantly whenever you run an application, because Windows NT itself needs about 12-16 MB to run. The PC can't hold NT and the application you are trying to use in memory at the same time, and it will thrash wildly swapping system tasks and application tasks back and forth from the hard disk. In a real system, it's important to carefully manage how the total memory in the system is used.
Some systems try to improve performance through the use of disk caching.
As they say, the road to performance hell is often paved with good intentions. In some cases, the system will reserve so much of system memory for caching disk accesses, that the remaining memory isn't enough and thrashing will occur. So you are using part of disk as virtual memory, and part of your memory as virtual disk. The amount of memory in a PC has a significant impact on its overall performance. Using too little RAM can be the biggest anchor dragging down overall system speed.