I just upgraded my old iMac (Mid 2010) to High Sierra which has a custom upgraded drive (Crucial 500GB). Running encrypted (FileVault) on previous Sierra and new High Sierra.

I am not seeing any difference in terms of read and write performance according to Blackmagic Disk Speed Test.

Solid 250 MB/S read and write for both HFS+ Encrypted and APFS Encrypted.

The caveat is that I know the iMac machine is old. I am in the process of upgrading my one month old 2017 MacBook Pro with Touchbar to High Sierra and will return with those results to see if indeed APFS encrypted shows a significant slowdown on newer hardware.