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I bought an external SSD with a USB 3.1 gen 2 enclosure. I'm on macOS 10.13.6 with a MBP (late 2013).

Using APFS I get write speeds of around 390–405 MB/s. So far, so good. However formatting the drive to APFS (encrypted) results in way slower write speeds of around 250–260 MB/s.

Is such a performance loss of nearly 40% normal for an encrypted drive, or am I doing something wrong?

I used a 10GB test file and cp to measure the performance. Copying through Finder results in similar performance.

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According to these benchmarks performances found here on APFS, APFS encrypted, HFS and HFS+ encrypted, your loss of nearly 40% may be surprising as you are writing and not reading.

Testing environment summary :

Hardware : 2017 MacBook Pro, 16 GB 2133 RAM

OS : macOS 10.13 High Sierra

enter image description here

Have you encrypted your already APFS formatted SSD to an APFS encrypted or have you completely reformated your APFS drive to an APFS encrypted ?

If you didn't reformat before your second performance speed test, this may be a cause.

Finally, if you did completely reformat your hard drive, this answer may be interesting :

Extremely slow write speed to encrypted external drive on Mavericks

  • Hm, but according to those benchmarks only the read speeds of APFS encryped should be significantly slower compared to normal APFS. Write speed should be about the same… – M.J.K Sep 11 '18 at 16:13
  • @M.J.K Indeed, I misunderstood your question. I have edited my answer with a possible answer : Have you encrypted your already APFS formatted SSD to an APFS encrypted or have you completely reformated your APFS to an APFS encrypted ? If you didn't reformat before your second performance speed test, this may be a cause. – J.Khamphousone Sep 11 '18 at 18:56
  • I completely reformatted the drive. I also tried encrypting the already APFS formatted drive but there wasn't any difference. So this shouldn't be the issue. – M.J.K Sep 11 '18 at 19:03
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    @M.J.K, ok, I edited a final time my answer and giving you an other Apple StackExchange link with a similar question of your and its answer. I know it's not a completely full answer and I hope the community will help, as it surpasses my knowledges ! – J.Khamphousone Sep 11 '18 at 19:12
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    Ok, I understand, have you tried to benchmark speed writing performance with HFS instead of APFS ? In case the performance with HFS are better, here are the pros and cons of HFS and APFS : techrepublic.com/article/… – J.Khamphousone Sep 11 '18 at 20:06

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