I recently upgraded a server from High Sierra to Mojave and decided to upgrade from ZFS 1.9.4 to 2.1.0
I could not figure out why writes and reads to encrypted data sets were dramatically slower. All the hardware was the exact same. Started troubleshooting my network. Then, just decided to test local copies from an SSD. However, both local and network writes and reads never went above 45 MB/s. I figured it must be Mojave or 2.1.0. Downgrading ZFS was the easier first step.
I uninstalled 2.1.0 and installed 1.9.4 and that instantly fixed the read/write performance. Over the network, the encrypted datasets are now saturating gigabit ethernet. And local writes from the SSD are +250 MB/s. I don't know if this a known issue or perhaps just particular to my setup. But if you too are experiencing really slow performance from you encrypted datasets, it might be 2.1.0
Though, 1.9.4's encrypted performance is so much faster that now I'm slightly worried my files aren't properly encrypted ; ) Maybe there's a reason why 2.1.0 is so much slower - it's actually encrypting the files.