Did a test last night based upon the description of your scene and my results are similar to yours. My scene had 500,000 particles and just simming the particles to disk took just under 3 hours for 240 frames. I didn't use HBatch, just straight ROP output driver. Houdini 11 FLIP solver does make particle fluids more practical to use.
Ian, thanks for posting your times so I have something to compare with.
cheers