It looks like stdio is five times slower than unistd:

% ./test.sh 4K 16M 64M 256M
Size 4K:
stdio:   0.00  456
unistd:  0.00  424
Size 16M:
stdio:   0.44  456
unistd:  0.08  428
Size 64M:
stdio:   1.75  456
unistd:  0.32  428
Size 256M:
stdio:   7.40  456
unistd:  1.32  428

Why?