Best way to genereate a terrain (top-down)

I am currently using arrays to generate my terrain, and am doing it 100x100 as i am learning.

I just generate tiles which are small (6x6)

But I will need my terrain to be around 1000x1000, and when i set the array to 200x200, it is a bit laggy.

So am i doing something wrong, or is there a better way to do it.

Thanks

Have you tried the tilemap object? https://www.scirra.com/manual/172/tilemap

Also Tiled is a great free tool. http://www.mapeditor.org/