Safari tabs stay active after closing

In my case, I kept seeing the 'spinning mouse/waiting' wheel behavior on my system. As it turns out, there were several processes running in the background too (after inspection of Activity Monitor), namely from 'web resources' and various URLs which ate up around 3-5% in CPU resources each. Yes, each!

The only thing I expected to see here that took up that much memory were actively running programs like Activity Monitor itself, Excel, Safari, etc. So, putting on my thinking cap I thought that maybe it could be these were still being detected by the system somehow even after Safari had been exited.

The resource hogs were due to various bookmarks on Safari (i.e. YouTube -- for which I linked to a half-dozen or so content creators) that were being a bottleneck to the overall system performance. Having since removed those bookmarks, the system is now running without a hitch. I was even surprised that was the root cause (as I have never seen bookmarks to be problematic), but it worked and this system woe is behind me, thankfully.

Note: As of this post, I haven't been able to repro it on-the-spot and I suspect it takes days or weeks on end that the system gets clogged up.