can't the grahics card be used to do (part of) the calculations to speed things up?

Unfortunately, the SunFlow rendering engine is not written to take advantage of GPU processing (if available). What's more, to date one cannot simply set a flag that says "use GPU" and have Java generate optimized code; instead, specific functions and routines most suitable for acceleration have to be identified, packaged for each of the two leading GPU libraries, handed to the GPU, executed, the results passed back to the main routines to be integrated with other data, and so on.

A while back, I glanced at some articles on potential improvements in ray tracing throughput from GPU processing, and they all said the same thing: Ray tracing, thanks to certain integral characteristics, is more resistant to reaping much improvement from GPU acceleration than other rendering methods. Before looking to the GPU for performance improvements, one should ensure the non-GPU code is as tight as can be,