Increasing simulation time in a Monte Carlo simulation in Matlab/Simulink

I'm running Monte Carlo simulation for a Simulink model with a Matlab script that looks more or less like this :

model = 'modelName';

load_system(model)

for ii = 1 : numberOfMC
    % Some set_param...
    % Some values are set

    sim(model);
    results{ii, 1} = numberOfMC;
    % ect...
end
close_system(model,0);

As the number of Monte Carlo trials increase, the time of one simulation increases as well like n^2.

Is there a simple explanation for that and is there a solution to have something linear in time?

Thank you!

EDIT:

When I split my simulation in 6 batchs, and I run them in series, the sum of the simulation times is far less than when I run the entire simulation ine one shot.