The CalculiX solver package on FreeBSD is compiled with the SPOOLES (SParse Object Oriented Linear Equations Solver) library by default. This is also the case in many Linux distributions because SPOOLES is free, while faster solvers like PARADISO are proprietary.

SPOOLES is relatively fast compared to the built-in iterative solver, its most fundamental limitation is that the data must fit in RAM.

However a patched version of the PaStiX solver has been integrated with CalculiX. It is often faster than SPOOLES and can use a GPU using the CUDA library.