Device: Samsung Tab A7 2020 - purchased less than 1 week ago

OS: Android 11, April 2021 patch

Files used to test: three zip files totalling 613 MB

Micro SD cards which have been tested:

  • Unbranded 256 GB
  • Sandisk Extreme Pro 128 GB

I have run CrystalDiskMark on the cards (in Windows over USB 3) and they have the performance you would expect; 70-90 MB per second.

Before Encryption:

  • From Windows 10 over USB 2 to SD card = approx 27 MB per second
  • From tablet internal storage to SD card = approx 56 MB per second
  • From SD card to tablet internal storage = approx 67 MB per second

After encryption:

  • From Windows 10 over USB to SD card = approx 1.2 MB per second
  • From Tablet internal storage to SD card = approx 0.2 MB per second
  • From SD card to tablet internal storage = approx 56 MB per second

That last one above isn't a typo - reading from the SD card does not seem to be affected by encryption.

If I format the cards to a state without encryption, the performace returns to normal.

I have tried doing a factory reset on the tablet - it had no effect.

Can anybody offer some insight into how to fix this?

  • Interesting. Most likely Samsung did not consider the smaller block size of common block ciphers and do write very small chunks of data. Flash memory is organized in large blocks of up to 256 KB which have to be written at the same time. Therefore if the block size decreases because of encryption this cloud explain the effect you observed. The the encryption itself is not a problem shows the reading speed. Encryption altos like AES have the same speed for de- and encryption.
    – Robert
    May 21 '21 at 18:12
  • Therefore I would assume this is a bug. Have you tested speed on device using a benchmark app? May be the problem is located in the USB MTP driver.
    – Robert
    May 21 '21 at 18:13
  • @Robert Thank you. I have not tried a benchmark app because honestly all the apps look like garbage. Does MTP still apply when copying data internally - i.e. internal storage to mounted SD card? May 21 '21 at 18:22
  • guesswork maybe it is encryption on demand only for used disk space (where initial encryption requires kinda additional formatting-like writing cycles)
    – alecxs
    May 21 '21 at 19:05
  • @alecxs That sounds reasonable - I have noticed that when the empty card is first encrypted by the tablet it completes instantaneously, which indicates it is not doing any kind of work in advance. May 21 '21 at 19:19

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