I think this is new behaviour since Safari 11, but I'm not absolutely certain.
I tend to leave Safari open all the time, with several pages of tabs, each for a different 'subject'.
On relaunch - if I've been cleaning less savoury cookies etc, or even that rare occasion I reboot - it used to reload all the tabs, whether currently visible or not.
This behaviour has changed & now each tab I go to will be blank & not load until I actually switch to it. I understand the philosophy might be to save resources until you 'prove' you actually need them, but honestly, if I didn't need them they wouldn't be open in the first place.
I've actually developed the habit of quickly flicking through all the tabs just after launch so I'm only seeing the irritating delay once.
Further irritation - the version it loads after the 2 second pause is still the cached version, so you have to immediately refresh it anyway:/
So I'd like to eliminate this behaviour.
I want, if possible, that each tab is already loaded with the current, not cached, version of the page at the time Safari was launched. I can handle my own refreshing after that, I just want to not have to wait that extra 2 seconds for each & every tab at first-use.
I have the Develop & Debug menus already visible, in case the 'fix' is in there, but frankly my comprehension of those menus is sub-optimal. I think in all honesty I have them visible to make me feel cool, rather than because I actually know what they're for ;)
This is currently Safari 11.0.2 on El Capitan.