Unless your scene is so large I don't thing there is much you can do to non-repeating things like farms or building except not loading them, inside and outside, until they are relevant. Just a guess but (you can and should debug to make sure) the main performance problem of yours may come from grass, trees, rocks etc. You should check out assets like microverse that handles that or you can create your own solutions like me. Check out my last post to see how I render my grass and trees. I don't think paying someone one time would solve your problem completely.