Recent HtoA 5.2.1 made it possible to actually use Arnold6 GPU in Houdini, i tool it on a test ride with particles. (5 million particles).
the setup is quite simple, pure diffuse shading only AA samples. I've chosen a darker frame to test the sample quality.
CPU 2 minutes on 6 core Xeon :
GPU 1 minute (Nvidia Quadro RTX5000), same sample count the CPU render above but more little more noise.
GPU 1:30 minute, increased the sample to get a noise free render.
the Arnold GPU render it getting slowly faster the Arnold CPU in some cases, there is still long road ahead to for the GPU renderer speed to catch up with competition.