Something similar to https://bugs.webkit.org/show_bug.cgi?id=193860 or https://bugs.webkit.org/show_bug.cgi?id=193215 but only main page case embedded <iframe> and that iframe uses history navigation. So swiping back freezes the page for a few seconds. Tapping back button is Safari works fine and the page is accessible immediately.
<rdar://problem/48496767>
Do you really mean iOS 12 Beta 3? (it was a thing, but many months ago). Your case sounds exactly like 239720; do you have a site that reproduces?
Oh, sorry, I meant iOS 12.2 Beta 3 :-( I don't have a repro though. I was just exploring putting our website (m.vk.com) into iframe and saw that issue. Not sure if it's specific to our website. It works fine when in main window though.
We can't seem to reproduce this issue. Please reopen if you can help us with additional test information.