This article explains why you might see a V-Ray GPU kernel compilation at the start of rendering.
Why does kernel compilation appear at the start of rendering?
V-Ray GPU compiles its rendering kernels once to optimize them for your exact GPU and driver version.
V-Ray GPU is forward compatible. The one-time compilation is needed so that our kernel is optimized for each driver version and GPU device.
When does kernel compilation happen?
Recompilation will happen only once when you install a new V-Ray version, if you change a driver or add a new GPU. In this case, it takes around 40 seconds for the CUDA engine and 45 seconds for the RTX engine.
Recompilation has to be done for every different GPU on the system. If you have several identical GPUs, chances are you will compile the kernels only once.
Do CPU devices (CUDA x86) require compilation?
No. CPU devices do not require kernel compilation.