The performance spike is due to pipeline state object compilation which happen when a combination of shader and state configuration are first encountered during runtime on modern graphics APIs such as DX12 and Vulkan. Note that this spike can be lower on subsequent runs of a player build depending on GPU vendors as a disk cache is sometimes handled at the driver level. Unity 6 introduced a solution to mitigate the issue, with the introduction of the GraphicsStateCollection API. On older versions of Unity, there is unfortunately no easy solution and we recommend using DX11 on Windows if this is an issue.