TeraBox pushes its Premium plan hard. Whether it's worth it depends entirely on what you actually use TeraBox for.
If you use TeraBox as your main cloud storage — backing up lots of your own photos and files — Premium's space and speed can be worth it. For that use case, buy it from the official app.
If you mostly just open videos people share with you, Premium is wasted money. The thing that frustrates you — slow downloads on shared links — can be solved for free:
Premium is worth it only if you're a heavy uploader. To download and watch shared links — which is what most people want — a free online tool does the job without a subscription.
It's worth it if you use TeraBox as your main cloud storage and need the extra space and upload speed. If you mainly download shared videos, it isn't — a free online tool handles that.
Yes. For shared links, a free online TeraBox downloader fetches the file at full speed without the free-account throttle, so you don't need Premium.
Mainly more storage, larger uploads, faster speeds and no ads — all aimed at people storing their own files, not at downloading shared links.