How to optimize the quantity of the foliage?

Hi, I used to add too many foliage and then caused the glitch problem,

so I had to reduce the density to less than 40%.

now the scene works smoothly, but only for a small area,
when I tried to apply the foliage to the whole area, same problems happened,

I’ve tried to reduce the lightmap from 16 to 4, but still not helpful,
is there any way I can optimize the foliage, so I can put more grass and trees into the game?