Copy files inside of windows and you will see the OS use multiple threads to do so. Granted its only as fast as the HD or RAM can handle it but it still uses CPU power to do so. In Photoshop the task is single threaded. I have files that literally take 2 or 3 minutes to save and they are less than 100 megs. Some are only 40 megs. Now save that same file as a .psd and the file will take about 10 seconds. I could be wrong but I thought any kind of compression was CPU based.