I have the impression that Mathics is currently not seriously concerned with performance. E.g., try this little microbenchmark in Mathics: "AbsoluteTiming[Sum[i, {i, 1, 100000}]]" or read their roadmap. This is fine -- there are many interesting applications of the Mathematica programming language where performance isn't important, e.g., carefully stepping through some symbolic operations with expressions. However, for Sage the motivation of most developers is cutting edge research math, and for that, performance is almost always very important. Performance is why Sage also implements a lot of similar functionality to Sympy rather than just using sympy for that functionality -- since sympy can be relatively slow due to their priority to be easy to install, which is definitely not a priority for SageMath.

The mission statement for SageMath is to be a viable alternative to Mathematica, Matlab, Magma, and Maple, but I never meant that to mean being a clone (e.g., directly running Mathematica code), but instead just an alternative in the sense that it can support research built on open source mathematical software that might otherwise be done with those closed source programs.