In my experience, btrfs send does a lot of random reads and is therefore rather slow when reading from mechanical disks. Backing up to a mechanical USB drive is fast enough, restoring from that drive can take forever (but you can use seeding). This tool reads the disk sequentially and should reach much higher throughput.

On the other hand, btrfs send skips empty and unchanged blocks. If the disk is mostly empty, send could still be the faster option.