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.