I've rebuilt my city world level (in my screenshot) 4 times now and I've got a fifth rebuild in order. Each time I do it, it gets better and it renders faster. My abandoned project has had at least 2 builds and a third one is needed. To rebuild mainly just the plants area shouldn't actually take long. Just have your model positioned and textured and use lots of copy paste (be sure you have plenty of copy paste on hand as it'll take quite a bit of it). For a whole field of grass, taking out the time to make textures, it should take but 5 minutes to cover an acre with an 8:1 map scale (8 quants is 1 foot) and, using a shortcut with texturing, applying the texture would take less than a minute. You don't have to do a complete redesign, just the part where all those plant entities are done.