how to reduce time of for loop?

hello, please i need a help i wrote a code that took an image and work on it to get certain metrics for her quality. the code is well running but the problem is with the time consumption it took 20sec can anyone help me please to reduce the time consumed by the for loop

the main purpose of the code is to calculate the max and the min of 3 consecutive values

img1=lum(img1); [M N]=size(img1);

e1=M-2; e2=N-2;

c=0;

for i=1:M-2 for j=1:N-2

        a=img1(i:i+2,j:j+2);
        a1=im2double(max(max(a)));
        a(a==0)=a1;
        b1=im2double(min(min(a)));

        if a1>0 && b1>0 && a1~=b1
        c1=(a1+b1)/(a1-b1);
        c_log=log(single(c1));
        c = c + (1/sqrt(c_log));
        end

    end
end

thank you