Size of a mesh vs performance

Hi everyone,

I'm trying to optimize every details of one of the mobile game I'm working on. I have a problem that I can't really understand but may be I miss something.

I actually check the profiler of my game to see which mesh is actually taking the most of time to render. My surprise was to see that the "heavier" mesh was ... the ground.

To be simple : We have a big 2tri plane mesh to make a "ground" in our game, with a colored material attached. This big 2tri plane is actually taking a lot of time into the profiler.

1) Do you know why ?

2) What I can do to optimize this ?

Thanks a lot :)