Slow write speeds with APFS encryped external SSD 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.
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



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