@KitRedgrave venture into the sources if you dare:
https://git.savannah.gnu.org/cgit/coreutils.git/tree/src/seq.c

even more horrifying, the implementation of yes(1) which prints out either y or a single string indefinitely
https://git.savannah.gnu.org/cgit/coreutils.git/tree/src/yes.c

i don't have any other implementations to look at right now, but i'd imagine it's not doing buffer allocation to print strings fast