I was doing one of those occasional backup of my root SSD to HD. They
are identical size, so sector by sector. I took this occasion to see if
blocksize (bs=) matters.
for i in {4,16,64,256}K {1,4,16,64}M; do
echo ==== $i ====
dd if=/dev/sda of=/dev/sdi bs=$i
sync
done
No difference. All did 117MB/s.