I'm running into an interesting result -- on my machine, the fast version runs at the same speed at LEN=1000000 (the default in the sample), but starts running

I'm running into an interesting result -- on my machine, the fast version runs at the same speed at LEN=1000000 (the default in the sample), but starts running 1.5 times faster at LEN=500000 and ends up twice as fast at LEN=100000 and lower. This is with gcc -O3. Why could this be?