How does `yes` write to file so quickly?

Let me give an example: $ timeout 1 yes "GNU" > file1 $ wc -l file1 11504640 file1 $ for ((sec0=date +%S;sec> file2; done $ wc -l file2 1953 file2 Here you can see that the command yes writes 11504640 lines in a second while I can write only 1953 lines in 5 seconds using bash's for and echo. As suggested in the comments, there are various tricks to make it more efficient but none come close to matching the speed of yes: $ ( while :; do echo "GNU" >> file3; done) & pid=$! ; sleep 1 ; kill $pid 3054 $ wc -l file3 19596 file3 $ timeout 1 bash -c 'while true; do echo "GNU" >> file4; done' $ wc -l file4 18912 file4 These can write up to 20 thousand lines in a second. And they can be further improved to: $ timeout 1 bash -c 'while true; do echo "GNU"; done >> file5' $ wc -l file5 34517 file5 $ ( while :; do echo "GNU"; done >> file6 ) & pid=$! ; sleep 1 ; kill $pid 5690 $ wc -l file6 40961 file6 These get us up to 40 thousand lines in a second. Better, but still a far cry from yes which can write about 11 million lines in a second! So, **how does yes write to file so quickly?**