Let's say you closed the browser with 5 tabs open. When you restart it, only 1 of them will be in the foreground (active), and that's the only one that's loaded. The background tabs (i.e. the other 4) are only loaded when clicked. Some people prefer this to speed up browser launch time and to save memory. I'm not actually sure which way it's does now that there isn't a choice (they must have picked one way or the other).
I think this is what Slimjet still does by default.
It's one of the reasons I found out about Slimjet in the first place (since I keep dozens of tabs open at all times, often around 100).
I just checked and it's still bevahing this way for me on 29.0.2.0 (tabs never pre-load until I explicitly open them), so if I close Slimjet and reopen it, only the first focused tab will load.