Before revision: General matrix-matrix multiplications (GEMM ) in vendor-supplied BLAS libraries are best optimized for square matrices but often show bad performance for tall & skinny matrices, which are much taller than wide.
After revision: General matrix-matrix multiplications with double-precision real and complex entries (DGEMM and ZGEMM ) in vendor-supplied BLAS libraries are best optimized for square matrices but often show bad performance for tall & skinny matrices, which are much taller than wide.