why vectorized code is slower?

I am trying to make my image processing program faster.

So, i changed my  scalar code into vectorized code.

for understanding, the purpose of program is" read right(4) and left(4) total 8 pixel of target pixel   from input buffer

then compare them with target pixel and calculate weight, then write result into other buffer.

so, i coded in this style

Read 16 pixel(Read_in) , then 8 pixel of this will be 8 target pixel(center).

After that,   split pixels into 4(letf)&4(right) . and store them into vector variables.

float8 splited1=(float8)(Readin.s0123,Readin.s5678)

float8 splited2=(float8)(Readin.s1234,Readin.s6789) and so on...

then compare splited1~n with center  by using vector operators and calculate weight.

Fianally, store result data(float8) into buffer.

In mali optimization guide, vectorized code is faster than scalar code.

but, in my case, vectorized code is slower than scalar code about 3 times.

why this thing is happened? is it caused by too many elements?

My device is samssung galaxy s6  equipped with mali t760-mp8