Thank you!
Yeah, it is because of the screenshots. It's something I'm continuously working on. It's surprisingly hard to get right. It should only take screenshots of loaded tabs, but it does indeed do it every time you focus the panorama view tab. The ~1.2 MB spike per tab is sadly expected here because Firefox needs to store the raw screenshot for a moment before I scale it down, compress and store it. I hope I can make this better in the future.
How pinned tabs work will be redone.
I will make the group scrollable when there are a lot of tabs too.

Thank you for the feedback!