Combine children and meshes. 

It is not written anywhere but mobiles don’t have the power as PC

I disagree. If you look at cpu alone you will find mobile cpus surprisingly high in the list.

https://www.cpubenchmark.net/cross-platform.html

But graphics are mostly done in the gpu and the gpu of a mobile is optimised as well.

What I am trying to say is this: games on pc will have settings to make them run on low end desktops. If they create the game so only 1% of the desktop pcs can run it, they miss out on potential customers. Also there are laptops.

Combine that with the lower resolution of a mobile screen and a slower frame rate and you will have mobile games seemingly outperform their desktop version. Seemingly. Going down from 4k resolution to 1/8 to 1/16 of the resolution and cutting frame rate by /2 or /4 and suddenly you only need 1/20th of the computing power. And this is not even considering reducing the bells and whistles of the graphics. Only bare bone pixels per second.

tl;dr Games will be optimised for the platform they run on.

Having performance issues with just 400k triangles in mobile signals something is not correct, mobile can do a few million vertices smoothly if done right.

The biggest issue on a mobile's GPU is memory bandwidth, which can explain your issue depending on how you are rendering those 400k triangles.

Batching (combining those 100 objects into one) won't solve the problem here, as GPU still has to go through all those vertices because it doesn't know they are just a copy of the 1 of 100.

What you want is to tell the GPU that you have 100 copies of the same vertex buffer, that's called Geometry Instancing (or just instancing), and it's largely supported by mobile devices nowadays. Keep in mind that for this you will need its own shader, I recommend also you calculate your object position on the fly, as sending 100 matrices to the shader in mobile may not be as fast as simply procedurally positioning them yourself.