I always seemed to have trouble with certain sites especially advertising ones. My main machine worked fine and my backup phone.
At first I thought it was content blocking. Or maybe cross site tracking. Then I cleared the history.
Turns out it was Safari Experimental Features. I had wandered in there and enabled them all. This is a bad idea. Causes some sites to hang.
Looking at a vanilla iOS 12 build. Make sure you only have in Setting/Safari/Advanced/Experimental Features checked.
Asynchronous Frame Scrolling
I dint know exactly what these mean or which one is breaking sites but this seems to work.
— Read on talk.tidbits.com/t/which-ios-safari-experimental-features-to-enable/6280