MacOS – Computer slows when battery is low

The process to enable FileVault will read the entire 500 GB of data - whether the block is empty or full and encrypt it with the keys you set up as part of the process. So, the background IO will run the fastest if you don't have other user level disk IO running. The encryption itself will take less than 10% of one CPU on that powerful (fast) Mac - so you are really just going to see a sustained 60 to 80 MB/s re-write of the entire drive if you let the Mac sit idle. A Mac with a spinning hard drive would see between 20 to 30 MB/s so an Air or any Mac with solid state drives will be two to three times faster in this operation.

So - from the time you start, I would estimate 2-3 hours if you are getting at least 70 MB/s for writing the encrypted data back to the disk. This process does run in the background and isn't really reversible once it starts, so you can kick it off and then track the progress with diskutil.

  diskutil cs list

Your best bet would be to use ControlPlane (free) to detect when the power source has switched, and then fire off a shell script to change the resolution.

There's no built-in command line utility for changing the screen resolution, but someone came up with one that works well. You can download an installer here.

Then it's just a simple matter of setting ControlPlane to run screenresolution set 1440x900 or 1650x1050 as appropriate. Note that you may want to experiment with the screenresolution get command first to double check how it handles retina resolutions.