How can i add large num of foliage without losing much performance

I was making an open world map but whenever I start adding large numbers of foliage like grass or rocks the FPS drops so low even without covering the whole map.
is there a way to optimize the foliage other than LOD ?

Not sure about world composition, but in the usual sense the tools you have at your disposal are:

  1. Grass type ( usually works better than foliage tool )

  2. LODs ( but don’t forgot to use billboard LODs if you’re working on that scale )

  3. Level streaming ( so you can swap out whole sections when you’re not actually ‘on’ them ).

Just to add to what Clockwork Ocean said.

You also want to make sure you have culling on the foliage meshes as well. You can set this pretty high and still get good performance.
It’s also worth nothing that for a good transition, you should use a PerInstanceFade node in your foliage material if you haven’t already.