Very good. I didn't benchmark it myself, but the language is designed for high-performance computing, and the authors' PLDI paper [1] shows it is competitive. Given that at the lowest levels, Terra basically reimplements C, and both compile to LLVM IR, similar performance is unsurprising to me.

The main point that gives Terra an edge is that its macro system makes implementing and testing potential optimisations easier.

[1]: http://terralang.org/pldi071-devito.pdf § 6. Evaluation