I found previous version was quite slow (3MB/s), but this newer Pi has more power.
What speeds can I expect with external hdd and standard installation of luks (I believe aes 256, xts, plain 64)?
Raspberry Pi Stack Exchange is a question and answer site for users and developers of hardware and software for Raspberry Pi. It only takes a minute to sign up.Sign up to join this community
I can see this question, the performance of Rasperry Pi 4 on LUKS, is old, but it is more relevant now than ever.
It is quite common to use LUKS with AES because CPUs (like Intel's, AMD's, etc) include AES acceleration in the CPU. This makes AES fast because encryption and decryption is performed by hardware. On the other hand, Raspberry Pi CPU's do not include AES acceleration, and this makes AES slow.
The situation has changed recently wih the creation of Adiantum encryption algorithm and its incorporation in Raspberry Pi OS's kernel modules. Adiantum was designed to be secure and fast in software, faster than AES in software. This allows LUKS on Raspberry Pi to be faster than before if Adiantum is used.
If someone wants to try it, this is a good guide that explains how to encrypt the SD card of a Raspberry Pi to use LUKS with Adiantum: https://rr-developer.github.io/LUKS-on-Raspberry-Pi/
The Pi 4 has USB 3.0, which I assume is what you plan to connect your external HDD over. The maximum bandwidth of USB 3.0 is 640 MB/s. (The Pi 3 had USB 2.0, which maxes out at 60 MB/s.) However, this is only the theoretical maximum speed.
It's hard to estimate the speeds you would get in practice, since software overhead will slow your R/W speed down.