'And with ten or fifteen open tabs it eventually becomes sluggish as hell.'
I don't think that's the standard use case for testing, nor should it be. What the hell are you doing with that many tabs open.
"Unfortunately, modern browsers are so stupid that they reload all the tabs when you restart them. Which takes ages if you have a hundred of tabs."
Again, good lord. Hundreds of tabs? What are you even doing.
As to refresh, I think that's become a user expectation that you see the most recent information when you pull up a tab. Having to manually do it isn't something a standard user is going to do.
Maybe what you're looking for is to have 'power user' settings in the browser, so you can keep your hundred tabs open.
I normally have perhaps 30~40 pages live (I don't like tabs, I prefer new windows) when surfing in an ordinary fashion. From time to time, I might even get up to a hundred. My browser does work fine, or to put it another way, roughly the same as when there are only one or two pages open.
What am I doing? Simple ... I refuse to be interrupted by crap. So when I am reading a news story (for example) I stay on that news story page and read the whole thing, then close the window (or tab, if you prefer). And if there is a phrase or event mentioned on that news page, I copy and paste into a new browser window to search that term and have it ready to refer to next, or later, or whenever.
Similarly when I'm researching a topic, I stay on the page I'm reading, and again will probably have a number of new pages rendered and ready for when I am good and ready to read them, in their entirety, as topics, phrases, pdf's, links, etc come into the picture.
Honestly I can't understand why someone would NOT have dozens of pages open at the same time. What do you do ... jump to a new page and leave the one you were supposedly interested in reading?