the script works normally for few data but if it accepts a matrix with a lot of data it takes too long time - matlab

Question

That script(matlab) works normally and fast for few data but if it accepts a matrix with a lot of data it takes too long time >1hr it run with no error but very very slow. What is the reason and what i can do for that (what are the main factors that make a program slow)
clear;clc;close all;
pref=input('Please enter file prefix: ','s');
fn=[pref '.mhk'];
fr=fopen(fn,'r');
k=0;
error = false;
t = fgetl(fr);
XTH = [];
Yed = [];
while ~feof(fr)
t = fgetl(fr);
[temp,~]=sscanf(t,'%f');
if ~error
k = k + 1;
XTH(k) = temp(1);
Yed(k) = temp(2);
end
end
x = [XTH(1)];
y = [Yed(1)];
XTH=[x XTH];
Yed = transpose([y Yed]);
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
n = length(XTH);
l = 0;
k = 0;
Vmin = 100000000;
r=0;
for u1 = 1:XTH(end)
l = l + 1;
if ismember(u1,XTH)==1
u1 = u1 + 0.1;
end
if u1>=XTH(1)+1 && u1< XTH(end)-1
for u2 = u1:XTH(end)
k = k + 1;
if u2 == u1
u2 = u2 + 1;
end
if ismember(u2,XTH)
u2 = u2 + 1;
end
XTH = [(XTH) u1 u2];
XTH = sort(XTH);
num1 = find(XTH==u1);
num2 = find(XTH==u2);
XTH = XTH(XTH~=u1);
XTH = XTH(XTH~=u2);
a11 = XTH(1:num1);
a12 = repelem(u1,length(XTH)-num1);
a1 = transpose([a11, a12]);
a2 = ones(n,1);
a31 = transpose(zeros(num1,1));
a32 = XTH(num1+1:num2-1)-u1;
a33 = repelem(u2-u1,length(XTH(num2:end)));
a3 = transpose([a31 a32 a33]);
a41 = transpose(zeros(num2-1,1));
a42 = XTH(num2:end)-u2;
a4 = transpose([a41 a42]);
A = [a1 a2 a3 a4];
AtA = transpose(A)*A;
B = Yed;
AtB = transpose(A)*B;
if det(AtA)==0
continue
end
if det(AtA)<1e-3
continue
end
solution = linsolve(AtA, AtB);
alpha1 = solution(1,1);
beta1 = solution(2,1);
alpha2 = solution(3,1);
alpha3 = solution(4,1);
beta2 = (alpha1*u1) + beta1 - (alpha2*u1);
beta3 = (alpha1*u1) + beta1 + alpha2*(u2-u1) - (alpha3*u2);
dy1 = 0;
dy2 = 0;
dy3 = 0;
Dx = XTH(end)-XTH(1);
width = 10;
for j = 1:num1
dy1 = dy1 + abs((Yed(j)-(alpha1*XTH(j)+beta1)));
end
for j = num1:num2-1
dy2 = dy2 + abs((Yed(j) - (alpha2*XTH(j)+beta2)));
end
for j = num2:n
dy3 = dy3 + abs((Yed(j) - (alpha2*XTH(j)+beta2)));
end
V(l+k) = Dx*width*(1/length(Yed))*(dy1+dy2+dy3);
if V(l+k) < Vmin && V(l+k)~=0
aa1 = alpha1;
aa2 = alpha2;
aa3 = alpha3;
bb1 = beta1;
bb2 = beta2;
bb3 = beta3;
U1 = u1;
U2 = u2;
Vmin = V(l+k);
end
end
end
end
plot(transpose(XTH), Yed, 'k')
daspect([10,1,1]);
% ax = gca;
% ax.XLim = [XTH(1) XTH(end)];
hold on;
grid on;
x11 = XTH(1);
x21 = U1;
y11 = aa1*x11 + bb1;
y21 = aa1*x21 + bb1;
plot([x11,x21],[y11,y21], 'r')
x21 = U1;
x22 = U2;
y21 = aa2*x21 + bb2;
y22 = aa2*x22 + bb2;
plot([x21,x22],[y21,y22], 'r')
x31 = U2;
x32 = XTH(end);
y31 = aa3*x31 + bb3;
y32 = aa3*x32 + bb3;
plot([x31,x32],[y31,y32], 'r')
%line(U1,aa1*U1 + bb1)
disp('Line equation 1:')
line1 = ['y = ', num2str(aa1),'*x + ',num2str(bb1)];
disp(line1)
disp('Line equation 2:')
line2 = ['y = ', num2str(aa2),'*x + ',num2str(bb2)];
disp(line2)
line3 = ['y = ', num2str(aa3),'*x + ',num2str(bb3)];
disp(line3);
vol = [num2str(Vmin),' m^3'];
disp('Minimum volume achieved:')
disp(vol)