Thanks for contacting us. Forest Pack creates natives instances in the render engine for each scattered object. Although the process is fast and efficient, it consumes more memory if the total number of objects is excessive.
For large areas we recommend creating patches of grass rather than individual plants. The Realistic Grass
tutorial will help to explain this, It's a bit old now, but the principles still apply.
If you're using our built in presets you'll see there are 2 versions for each: Detail and Field. Make sure you choose the latter if you're scattering on large areas, these use the same "patch" principle
I hope that helps, please let me know if you have any further questions.