The program is too slow

I have data that I process with my code, but this data is too big and my program does not work, but for a smaller amount of data, it works well. How do I make the program work for my large amount of data? My data is so big that I can't attach it

My code:

gn_nach(:,1) = tv;
gn_nach(:,2) = dv;
format longG
d_first = 0.3584*(2/3)-0.11458-0.00415;%0.408/sqrt(3)-0.1+0.00172;%0.183908709293856-0.18; % d for first element   0.0522230368891257   0.05149276   0.070729999
d_rest = 0.3584*(2/3); % d for other elements  0.4053
ii1 = 1; %%%%%
ii = 1; %%%%%%
a_0 = 0;
%a_0 = optimvar('a_0',43521); %!!!!!!!!!!!!!!!
general_t_d(:,1) = gn_nach(:,1);
general_t_d(:,2) = gn_nach(:,2);
% Check if first element
if ii1 == 1
    d = d_first;
else
    d = d_rest;
end
while ii < size(general_t_d,1)
    if abs(general_t_d(ii+1,2) - general_t_d(ii,2)) < d
        general_t_d(ii+1,:) = [];
    else
        ii = ii + 1;
        d = d_rest; % change d value after first element check
    end
end
disp(general_t_d)
c1 = 1;
c2 = exp((-(3/6)^2)/2);
c3 = exp((-(6/6)^2)/2);
c4 = exp((-(9/6)^2)/2);
c5 = exp((-(12/6)^2)/2);
c1 = 1;
c2 = exp((-(3/6)^2)/2);
c3 = exp((-(6/6)^2)/2);
c4 = exp((-(9/6)^2)/2);
c5 = exp((-(12/6)^2)/2);
c6 = exp((-(10/6)^2)/2);
c7 = exp((-(12/6)^2)/2);
%c8 = exp((-(10/6)^2)/2);
%c9 = exp((-(12/6)^2)/2);
k = c1 + 2*c2+ 2*c3 + 2*c4 + 2*c5;% + 2*c6 + 2*c7;
c1 = 1/k;
c2 = c2/k;
c3 = c3/k;
c4 = c4/k; 
c5 = c5/k;
%c6 = c6/k; 
%c7 = c7/k;
%c8 = c8/k; 
%c9 = c9/k;
C_sum = c1 + 2*(c2+ c3 + c4 + c5);% + c6 + c7);
t_v = general_t_d(:,1);
d_v = general_t_d(:,2); 
%y_res(1) = t_v(1);
%y_res(2) = t_v(2);
%y_res(3) = t_v(3);
for i = 6:length(t_v) %9
    if i == length(t_v)
        y_res(length(t_v)) = t_v(length(t_v));
    elseif i == length(t_v)-1
        y_res(length(t_v)-1) = t_v(length(t_v)-1);
    elseif i == length(t_v)-2
        y_res(length(t_v)-2) = t_v(length(t_v)-2);
        
    elseif i == length(t_v)-3
        y_res(length(t_v)-3) = t_v(length(t_v)-3);
    elseif i == length(t_v)-4
        y_res(length(t_v)-4) = t_v(length(t_v)-4);
    elseif i == length(t_v)-5
        y_res(length(t_v)-5) = t_v(length(t_v)-5);
        %{
    elseif i == length(t_v)-6
        y_res(length(t_v)-6) = t_v(length(t_v)-6);
    elseif i == length(t_v)-7
        y_res(length(t_v)-7) = t_v(length(t_v)-7);
        %}
    else
        y_res(i) = c1.*t_v(i) + c2.*t_v(i+1) + c2.*t_v(i-1)+ c3.*t_v(i+2) + c3.*t_v(i-2) + c4.*t_v(i+3) + c4.*t_v(i-3) + c5.*t_v(i+4) + c5.*t_v(i-4);% + c6.*t_v(i+5) + c6.*t_v(i-5) + c7.*t_v(i+6) + c7.*t_v(i-6);  % + c8.*t_v(i+7) + c8.*t_v(i-7) + c9.*t_v(i+8) + c9.*t_v(i-8);
    end
end
figure('Color','w')
hold on
plot(d_v,y_res,'-r')
grid
plot(dv,tv,'-b',d_v,t_v,'.black')

[Walter Roberson]

preallocate y_res

[Dyuman Joshi]

In addition to Walter's comment (which you should consider as a priority to improve code efficiency) -

Is the variable gn_nach used anywhere else in the code? If not, you should avoid defining it and directly define general_t_d, as copying of large amount of data can slow your code as well.

Additionally, you can also club all the initial if-elseif statements -

l = length(t_v);
if ismember(i, l-5:l)
    y_res(i) = t_v(i);
else
    y_res(i) = %formula
end