What happened to the performance of the 'back' button? Pressing 'back' used to be quick, on the modern web its rather sluggish, like it re-loads the old page f

What happened to the performance of the 'back' button? Pressing 'back' used to be quick, on the modern web its rather sluggish, like it re-loads the old page from scratch. I can forgive sites that are implemented as single page with their own 'fake' URLs (though that is still annoying). But even static content. Given the emphasis on browsers handling lots of tabs I'm surprised 'back' isn't implemented by, effectively, keeping the old page around for a bit and pausing its Javascript, then just picking up where you left off.