I’m installing Debian on a computer where the rootfs will be stored on two 250GB NVMe drives that are RAIDed (RAID1) together. I couldn’t figure out a good way to also RAID (and add some redundancy to) the ESP partition, unfortunately, and I don’t think that’s possible – though it would be cool to have a working ESP partition, no matter which drive might fail. In the end, I followed this tutorial and RAIDed together two 249GB partitions I made on each on the drives – using the TUI (ncurses) interface of the Debian installer. Then, I decided to not install the system until mdadm would finish the sync, did Ctrl-Alt-F2 to switch to a terminal, then did cat /proc/mdstat – only to see 1000K/s speed and “30 hours left” estimate. Given that the drives were NVMe, this was very weird.
However, it seems like some settings in the Debian installer environment artificially limit the RAID sync speed to 1000K/s. Following this tutorial, I removed the limit using this command:
echo 1000000 > /proc/sys/dev/raid/speed_limit_max
Then, all went well and the array synced at full speed (1G/s in this case). Hope this helps you too!