I'm not completely certain but here's my stab at it. Windows is a multiprocessing system, which means it switches execution from process to process fast enough to look like its happening simultaneously. Whenever the CPU switches from one task to another it does someting called a context switch where it saves all the information of the current process (ie. the state of the process) and loads the next process's state. When there is a large amount of information to be stored this context switch becomes noticeable.