When you load scenery for the first time under Vulkan, X-Plane undertakes a lengthy process to compile shaders for it. You end up taking that hit when you switch seasons, at least for the first time.
15 minutes sounds excessive however, and this sounds like it may be a bug in the X-Plane Vulkan beta. We are using X-Plane's API's for reloading scenery (XPLMReloadScenery) and my guess is that it doesn't work as expected under Vulkan. I'll file a bug report with Laminar about it.
For now, you should stick with OpenGL with TerraMaxx or use the workaround you identified.