Same for me. I found some suggestions to turn on/off Speed loading, and found opera://flags/#use-angle and turned on and off and changed several settings about Angle and for rendering. But all result in the same: pages load very slow, up to minutes.
I'm on an AMD CPU and AMD GPU. Are there optimal render setting for that combination?
Also, before a few months ago Opera was quite fast. So it may have started with an update in the past six months or so.