I have been encrypting a number of external disk using FileVault and have noticed puzzling behavior. The disks are external USB 3.0 disks.
When the process starts, I can see that the disk access proceeds at a reasonably snappy speed of ~35 MB/s using Activity Monitor.
If the computer is not touched for a while, the disk speed drops to a paltry ~5 MB/sec and will stay that way. As soon as I touch the mouse or press a key, it jumps back to the ~35 MB/sec speed and will stay that way until it goes idle again.
I would have expected just the opposite behavior. Keeping the disk accesses low while the user is active provides a snappy UI experience. Once the user is inactive, full bore access would seem to be the thing to do. Can anyone explain this behavior?
An example is shown here. Active disk while interacting with the computer, then slow disk after no interaction. As soon as the mouse was wiggled, the speed went back up.