Vertex normal utilization

I have a 3D model, which is separated peice by peice.
When I load it up, it appears as normal.
But I thought I could improve the rendering performance by merging those into one

so that I’ve merged those into one and load it up then the normal gets broken.

And I found that there’s no significant performance improvement although I’ve merged those into one :frowning:

What can be the problem here?

You’ve just established that what you changed was not your main bottleneck.

I suggest you always profile first. Then optimize. That reduces the amount of time/effort/code spent reworking things that often won’t net you any gains.