Usually the GPU process. Even if your tabs are all inactive, killed by something like Great Discarder, the GPU process continues to use a lot of RAM (2GB in my case, at times) and CPU.
I am not sure if it's a Mac-specific issue though.
I literally cannot comprehend why anyone would use bookmarks when the friction is multiple orders of magnitude higher.
99% of my tabs do not live longer than 10 minutes so why should I want to persist them in a messy non hierarchical list that is neither organized by context nor by time? If you somehow make bookmarks as frictionless as tabs by making the new tab button create a bookmark and then show that bookmark as a tree structure on the sidebar then it wouldn't be any meaningfully different from a tab other than the fact that it has to reload the entire page every time you switch bookmarks.
AFAIK, Firefox has countered this by only loading the tabs when you activate them after a cold start, so that's pretty much automated bookmarking.
I wouldn't know how to manage without bookmarks (well, yes, .url files actually, but that's another issue). I'm guessing I have a few hundred of them, and I find what I need easily, because you can organize them in folders.
Huh? Those have not much to do with whether some tab is open or not. Pages that have reasons to care about seeing length (and many which don't have good reasons) will log you out anyway.
You're right, cross-site scripting is the wrong word, sorry... what do you call it what the Spectre-exploit does? It was the case that the exploit could access data from the other tab via Javascript, IIRC.