Not sure if anyone else is having this issue or if it's normal. When I try to preview a render with XPU it takes a couple seconds to initialize the optix devices. As opposed to rendering with CPU when the render will start right away.
If I make certain changes in the shader like adding a new node it has to re-initalize. Once the optix devices start rendering it's very fast but constant initializing makes it slow to use and not a very interactive experience.

Perhaps a Optix issue? I'm not sure. I've tried installing the latest Nvidia drivers. I'm using a RTX 3070 + GTX 1080.

Here are the XPU docs
https://www.sidefx.com/docs/houdini/solaris/karma_xpu.html [www.sidefx.com]

"...On startup, Karma XPU must compile shaders into device-specific formats. This process will cause the XPU to pause/stutter when you load a new scene or enable a new feature. The compiled results are cached, so rendering the same scene will not pause for shader compilation after the first time...

Do there is not much that can be done to further improve your experience with H19.5
But I can say that in upcoming versions of houdini, shader compile times for interactively editing nodegraphs will be faster.