Time complexity of Gauss-Seidel iterative inversion method

Hi,

What is the factor that causes Gauss-Seidel method takes longer time execution in MATLAB caompared to the neumann series method?

Is it because of the lower triangular matrix inversion that is solved using Forward-backward substitution? For your informtion, I use tic-toc function in MATLAB to measure the elapsed time required by both method.

For example, with 1 iteration, GS takes 1405 sec while Neumann series takes only 138 sec to be executed.

Thank you.