Hi,
I'm sorry to hear that you were facing these troubles. First, let me mention that generally, we suggest using polygon or mesh models. Internally, Forest converts all items to native instances of the render engine, making it as efficient as using proxies. You can find a detailed technical explanation
here. Also, using XRefs would be recommendable in many situations. Please feel free to check this "
Best Workflow for a very large scene with Forest!" thread, where you can find some more tips for dealing with heavy objects/scenes. I hope you'll find it interesting.
I would suggest you check
this post. It might be related - you should optimize your V-Ray proxies to be rendered as instances. If not, V-Ray takes a long time and memory to load and unload them.
Thanks in advance for any further comments on this issue you can share about it with us/our user community (in case some user(s) will run into similar troubles).
Best regards,