I am also seeing this in Opera on Mac OS. I've seen it for quite a while, but it seems to be worse lately.
Behavior I see: Clink on a link and page begins to load and spinner shows on the tab header. The connection state shows that the certificate is loaded and trusted at this point - so the TCP connection has succeeded and the TLS handshake apparently succeeded (not 100% sure, it would have the cert at the beginning of the handshake process, so it's possible that that being present in the UI does not indicate handshake completion). At this point, the tab is blank, and then after some time the page suddenly pops and gets rendered. I think this is normally 5-10s, but I counted it earlier today and it was more like 40s, which led me back here (I had checked here in the past, but the hassle of creating an account was enough to stop me from reporting).
I have ad block and tracker block disabled. I have no extensions installed. Opera is up to date
Any suggestions for getting more diagnostic info when it's in this state to figure out what it's waiting for?