Using the GPU probably won’t help because you’re just writing a file to disk and not performing any transformations (unless you’re transforming some data and then writing it disk, and measuring the time it takes for that?)
3.5G with 2hr 49 min is around 450 kilobytes / second, which is slow for a disk. You can check if the problem is that your disk is slow via watching the output of iotop when you’re saving.