Thanks a lot for taking the time to review the extension and for the detailed feedback, I really appreciate it.
A bit of context: I know some JS, but very little about browser extensions. This is practically my second one, so I'm still new to all of this.
I'll look into the performance side. From what I've seen, the delay comes from reading a lot of data at once, and it only becomes noticeable with a large list. With a normal number of hidden games it doesn't have a real impact. Still, I'll review it to see if it can be improved.
About storage, I'd rather keep the permissions to the minimum for now. The extension only stores short ids and names locally, so I don't expect most people to get near the default limit. If it turns out to be a real problem, I'll reconsider.
Your comments were very technical, which is useful, but I'd also love to hear feedback from a user's point of view: how it feels to use, what's confusing, what's missing. That would help me a lot too.
Thanks again for the suggestions!