SymEngine is a fast symbolic manipulation library, written in C++.
Ondřej started the project in 2012, and since then developed a user and developer community. According to many benchmarks, SymEngine is one of the fastest computer algebra systems (whether open source or commercial).
SymEngine is written in pure C++, but has wrappers to many languages (Python, Julia, Ruby, …). It can optionally be used as a backend in SymPy.
Project link: https://github.com/symengine/symengine