If you have very little real memory your computer has to constantly write contents of your memory out to make room for new data. This old data is then stored on your hard disk. During such a process your computer can't do much of anything, it can't run the new program until the bits for that app has space in the memory to load to, which is in turn waiting for some old space to free up, which is in turn dependent on your hard disk drive. In microprocessor time a disk access is an extremely slow process, so you can just imagine how much of your computer's juice is spent waiting for data being written to disk.