Unfortunately the encoding of images to JPEG2000 (the compression scheme used by DCPs) is computationally intensive. The library we use for the compression (openjpeg) is very good, but a little slow compared to some (expensive and closed-source) commercial libraries.
I am hopeful that over time the performance of openjpeg will be increased, either by me or by other developers that work on it. There is also some movement towards the acceleration of openjpeg by using the processing power of graphics cards.
In the mean time, there are a few things you can do to increase conversion speed: