Encryption on Asus Fonepad FE170CG, or how to transform your phablet in something slow as a dog

I recently fixed a problem on an Asus Fonepad FE170CG. This tablet (or technically, more a phablet, since you can also make phone calls with it) is sold in Italy at the bargain price of 99€ and is quite popular in circles where people need a cheap appliance that can stand as access point, and make the occasional phone call.

Asus FE170CG fonepad
Asus  fonepad (courtesy asus.com)

The owner claimed an issue with the specific device, saying that it had become dead slow after a pair of months. I could not understand, as I personally used it for a couple of days after configuring it and found it a pleasant experience at a bargain price. The owner was quite pleased for a while, and keep telling me to buy it because it was just such a good device.
But when the user handed the device back after a pair of months, dead slow it, was, indeed! This puzzled me a bit, it was like using another device.  I made the usual checks, antivirus, rogue apps, issues with space and so on.  Hardware problem? The fonepad is plastic, but it is quite sturdy, and showed no signs of damage or mistreating. Then I recalled that some time ago I persuaded this specific user to encrypt the device, on the basis that mobile devices SHOULD be encrypted.  Could encryption  be the culprit? Unfortunately it is not possible to rollback encryption on the Fonepad (or in Android devices as far as I know), so the rollback required a backup and restore to factory setting.

I made a quick benchmark using AndroBench  before and after the restore. The results are of course inconclusive, as I cannot be sure that something else was wiped along with the encrypted image, and for this reason I won’t publish the screenshots I made in the process. But with the same apps (the restore process was quite painless, as the user used the Google backup/restore option) I had a x10-x30 improvement in speed, especially in SEQ RD and SEQ WR. Now for the reasons above it would not be fair to say that encryption was causing the terrible slowness the user lamented, but I believe it may be a realistic cause.

Maybe the Intel Atom processor has no specific hardware optimization for Android encryption? Maybe the processor on the FE170CG is simply too light to provide a good user experience with an encrypted storage? Maybe they did not put some proprietary drivers in the OS to use the specific hardware acceleration that the CPU provides?
Until I discover this, I think I will not advise encryption on these light devices. It’s no use having a more secure device if it’s slow as a dog, it’s better to have a less secure device, threat it as potentially compromised and keep private information on it at minimum. Or better yet, it’s better to buy a faster device in the first place if you want to use the encrypted storage.
This specific device is running fine and the user is again very happy with it. I am still a little bit concerned with all those private data going around un-encrypted, but at least the device is in use again.