FileVault initial encryption slowing down after computer idle for 15 mins?

I have a couple of external hard drives (on USB 3.0) that I'm in the process of encrypting using FileVault2 (Sierra 10.12.3 on MBP). So far, all is going fine, though as the drives are 2TB and 4TB things are taking some time (18 hours and counting), no surprises there.

What I *am* surprised by, is that FileVault goes into a "slow" mode if I walk away from my computer for 15 minutes or so. Now, I'm running on a power adapter, have Energy Saver preferences set to "Never Sleep" for both Computer and Display (though I can't see why display should matter), have "put hard disks to sleep when possible" off, and have screen saver set to Never. So I've done all I can to stop the computer going to sleep (it doesn't).

Despite this, if I simply stop using the mouse or keyboard then, after about 15 minutes, FileVault drops from maybe 30-40Mbytes/second of encrypting, to about 3-4Mbytes/second (as reported by Activity Monitor). It then stays there until i tap a key or shake the mouse, at which point it instantly ramps up to full speed again for another 15 mins.

This is pretty annoying, as my plan was to leave the computer working overnight etc to do the bulk of the encrypting, but with this behavior it's going to take 10x longer than expected, unless i sit up all night bashing the mouse.

Has anyone else seen this, and/or can anyone suggest a workaround? I'd love not to have to wait until spring for this task to finish 🙂

--Tim