How is parfor time taken is larger than that by normal for loop ?

The time taken (t1) by parfor is larger than that (t2) by the normal for loop,I don't understand why ?

Answer 1

[Sindar]

Best guess: you don't preallocate y, nor clear it. So, the parfor needs to expand the vector constantly while the for is simply overwriting elements. Try

x=1:4000;
y1=false(size(x));
y2=false(size(x));
...
    y1(i)=
...
    y2(i)=