The very first time you render with the GPU, the GPU renderer has to create a cache of shaders. This can delay the time to first pixel for your first render.
To avoid the one-time delay, we recommend that you run Pre-Populate GPU Cache before you do any renders. Note that pre-populating the cache can take up to 15 minutes.
The cache only needs to be re-populated after installing a new Arnold version, updating to a new NVIDIA driver, or changing the hardware configuration of GPUs on the system.
You can easily switch between CPU and GPU with a single click in the Render Settings > System.