I need to perform some heavy computations involving polynomials in many variables. I don't need any fancy features, just adding and multiplying polynomials and substituting variables by other polynomials. Ideally, I would need to define variables with multiple indices, such as $x_{3, 1, 4}$. I've tried SageMath but the polynomials are so large it is too slow. I'm looking for a faster alternative (perhaps some C++ library?) that also supports parallelism.
Do you have any suggestions on what software to use? Thank you very much.