At least for pyfr v1.12 you can manually set the cutoff number of non-zeros for gimmik with gimmik-max-nnz = 512 in the openmp backend section. Although this will have an effect on performance. If you have a look at the gimmik paper you can see how the performance changes with matrix size and sparsity.
The libxsmm library can also be an option, see the PyFR OpenMP documentation and the Libxsmm git.
PyFR will also fall back on a blas gemm if the operators are too dense or too big. You may want to check that you have a blas library in your library path. On WSL I think you can do sudo apt install libblas-dev for a quick install, or you can compile your own, or if you have it Intel’s MKL can be a more performant option.