You have great internet connection and video is buffering? Sadly there is nothing we can do about it. All content in the Stremio is provided by the system of Add-ons. You can "install" as many add-ons as you want because it is free to install and use them. You have to know that community add-ons are created by Stremio users therefore we don't control them.
If movie keeps buffering or you see information that source is poorly available it probably means that add-on doesn't provide good enough source. You can install more add-ons and use sources provided by different add-on. Also you can try to change your DNS. It might help a lot. Here you will find instructions how to do that https://blog.stremio.com/change-dns-resolution/