So converting will take a bit more time of your general cpus time. The whole point of converting your RAW on your computer is to fiddle a bit more with the parameters than available in your camera chip - which is specialized for converting fast and good. You might speed up by using multiple cores (you tried: parallel execution, if supported by the program) and a lot of ram (your os does: caching the files).
This won't be sped up significantly, regardless which program you use.
