When writing any data to SD cards, the operation is usually very slow compared to other kinds of media (up to tens ob MBytes/s). However, when using an encrypted file system on the SD card, such as encrypted APFS, the write operations tend to be even slower, for example one fourth of the transfer rate. Why is this the case? Modern CPUs should provide special instructions that are also used for any other file vault operation.
I suspect the encrypted vs unencrypted read and write speed has to do with continuous vs random performance on (Micro-) SD cards as measured and described here: https://www.jeffgeerling.com/blog/2019/raspberry-pi-microsd-card-performance-comparison-2019