Slow write speeds with APFS encryped external SSD
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