Why does it happen at the start of rendering?
V-Ray GPU is forward compatible. Kernel one-time compilation is needed so that our kernel is optimized for each driver version and GPU device.
When does it 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 will need 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.
What about CUDA X86?
CPU devices don't require kernel compilation.