40% drop is reasonable with the all the overhead of the encryption, it's not for free. However I have found out with the M1 airbook it is also drive dependant.
M1 airbook with a verified dongle (can push 100+ MB):
Transcend 1TB: unencryped 100-110 MB/s, encrypted: 40-70 MB/s
Seagate 5TB: unencryped 100-110 MB/s, encrypted: 15-20 MB/s
2015 macbook pro (without dongle):
Transcend 1TB: unencryped 100-110 MB/s, encrypted: 60-80 MB/s
Seagate 5TB: unencryped 100-110 MB/s, encrypted: 60-80 MB/s
As you can see the 2015 macbook trashes the M1 airbook here, but encryption is the key here, because if i create an unencrypted APFS volume, it's pushing
storagekitd using a lot of cpu on M1.
And I also managed to kernel panic M1 big sur just by connecting an external hard drive with
Suspected bundle: com.apple.iokit.IOUSBHostFamily.
This panic has big chance of happening whenever the notebook goes to sleep too:
USB is very raw on M1. Let's hope all the panic backtraces I sent will make a difference.