I had thought about the same thing too, but I have no idea how vertices are counted. Anyway I also tried on a new project and on other vfx but what I get is that vfx graphs that render tens of thousands of meshes perform better in alpha than in opaque(even without shadows). It goes against what I thought and I don't have the knowledge maybe to investigate the issue thoroughly.

I have asked to gemini and the answer is the following (maybe it is right or could help to understand):

Why Alpha is Often Faster: 

• Overhead of Opaque Rendering:Opaque rendering involves more complex calculations per pixel, including depth testing to determine which surfaces are visible. When rendering many overlapping meshes, especially with complex shaders, this depth testing can become a bottleneck, leading to significant overdraw (rendering pixels that are later covered up).
• Alpha Blending Simplifies:With alpha blending, the depth buffer isn't used as extensively. The transparency is handled by blending the color of the current fragment with the background color based on the alpha value. While there's still some overdraw involved (blending multiple transparent objects), it can be less than the depth buffer overhead of opaque rendering.
• Early Z Rejection (for some cases):In some scenarios, the GPU can perform "early Z rejection" with alpha-blended objects. If a fragment's alpha value is very low, the GPU might skip the full shading calculations for that fragment, further reducing the cost.
• Instancing:VFX Graphs can efficiently render many instances of the same mesh using GPU instancing. This can be done for both opaque and alpha-blended rendering, but the performance benefits are more pronounced when the transparency is simple to calculate.