If your hardware specs don’t suck, you might be using the wrong settings. The number of foliage instances can easily grow to thousands for a very simple forest (grass, trees, flowers, etc.) so every instance must be optimized.
First off, are you using the engine’s foliage types? do you paint them with the foliage painter and optimize draw distance / amount per instance? are you calculating shadows for all of them or not?
If you’d place that amount of plants as unoptimized hand placed static meshes I’d not be surprised at low performance on good hardware.