The very first time you render with the GPU, the GPU renderer has to create a cache for the types of shaders and objects used. This can delay the time to the first pixel for your first render. To avoid these one-time delays, we recommend doing some of this in advance by pre-populating the cache before you do any renders as this will fill in the cache with a subset of the most common shader/object combinations. The Arnold plugins (like MtoA) have Pre-Populate GPU Cache menu command, and kick has a -gpu_warm flag. 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 Arnold plugins, you'll find a new Render Device setting in the Render Settings > System section.
You can use