I've start test Houdini 18 and Arnold 6. the first test was simple splines rendering, 250.000 splines instanced 25 times. it loads a 140MB alembic file.
rendered in 6 core Xeon CPU and Nvidia Quadro RTX 5000. (windows 10 pro)
the startup for Arnold GPU is slow, it renders faster, so it seems but for clear up the final image it takes for forever or just dropped /crashed, hard to tell on the GPU. the CPU is quite fast but much slower then GPU if it ever would finish. (adaptive sampling was on)
As soon as Arnold finishes rendering the scene, it stops and do not refresh any more on parameter changes. so far i am not impressed with the Arnold GPU rendering.
here is the same scene Arnold CPU with only direct Lighting. (on my MacBook)
some test with Arnold GPU. it performed much better with just direct lighting.