How to reduce computional burden (symbolic expression)

I have written a MatLab code to build a polynomial expression that make a straight link between the output and input of a Neural Network algorithm (GMDH). For this purpose, I had to perform many symbolic polinomial substitution resulting in a very slow processing which many times ends up crashing the software.

Within the code I have a symbolic expression involving variables y1 and y2 and then each variable is replaced by another symbolic polynomial expression involving four others variabels: x1, x2, x3 and x4. This processes is executed many times and as the polynomial expression grows the proccess becomes slower. The code line applied for symbolic substitution is as follow:

digits (4)
MatrizPol(i,j) = vpa((subs(MatrizPol(i,j),y1,MatrizPol(i-1,cont_neuronio))));
MatrizPol(i,j) = vpa((subs(MatrizPol(i,j),y2,MatrizPol(i-1,cont_neuronio+1))));

I am looking for another function, algorithm or any other way to reduce the processing burden of this polynomial construction. Any help is appreciated, thanks in advance.