After you deal with parsing and hashes, basically you are IO limited, so mmap helps. The C code takes less than 1.4s without any CUDA access. Because there is no compute to speak of, other than parsing and a hashmap, a reasonable guess is that even for the optimal CUDA implementation, the starting of kernels and transfer of data to the GPU would likely add a noticeable bottleneck and make the optimal CUDA code slower than this pure C code.
After you deal with parsing and hashes, basically you are IO limited, so mmap helps. The C code takes less than 1.4s without any CUDA access. Because there is no compute to speak of, other than parsing and a hashmap, a reasonable guess is that even for the optimal CUDA implementation, the starting of kernels and transfer of data to the GPU would likely add a noticeable bottleneck and make the optimal CUDA code slower than this pure C code.