In doing GTCs, I found the largest time user was saving the result after each processing step (e.g. saving after calibration, after applying the orbit file, after multi-looking. etc.). The actual processing step would take only a few seconds, but the writing of the data to a geotiff would take anywhere from a few minutes to a few tens of minutes.
If you know that your processing steps are correct, I would suggest getting rid of every save step except for the final product.
Otherwise, if you can upgrade your hardware, I would also suggest installing SSDs. They're expensive, but much faster (and more durable).