Why is this Matrix Multiplication taking so long... using diag function

Hi the vectors are of length , and an array K is of length I want to turn into diagonal arrays to calculate the product and then sum the columns of this product, my code is

a=diag(a);
b=diag(b);
op=a*K*b;
p=sum(op);

but it takes a really long time to run, am I doing something that is bad practice which is slowing down this procedure im confused why this should take so long? More specifically the run time for that bit of code is 45 seconds !

EDIT : Consider the code

m=10000 % or something larger !
K=rand(m);
a=rand(m,1);
b=rand(m,1);

% I want to calculate the matrix which has component i,j $a_i K_(i,j) b_j $
% and then sum the columns of the resulting matrix. 

% The following code is how I did this but it takes 45 seconds to run, is
% there a faster method ?
tic
a=diag(a);
b=diag(b);
op=a*K*b;
p=sum(op);
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