Hi, I'm the author of fast_matrix_market, a package that reads and writes Matrix Market files: https://github.com/alugowski/fast_matrix_market/tree/main/python The Python binding is API compatible with scipy.io.mmio, but about 25x faster. A few other handy improvements include 64-bit indices (only if necessary), direct csc/csr writes with no coo intermediary, longdouble. It passes the SciPy mmio test suite. Would there be any interest in somehow integrating this library into SciPy? The speed difference really does make a big difference for large files. The package is written in C++17 and pybind11. Threading is with a simple thread pool based on c++11 threads, but can be changed. A significant part of the speed increase is parsing with std::from_chars instead of strto*, which alone contributes a massive improvement.