Hi @avl – first of all thank you very much for all the work you've put in to figure this out. I'm grateful for your help.
Maybe it's superstition, but I had noticed my 2013 Macbook Air was slowing down a bit and I didn't want to upgrade to Sierra in case it exacerbated that. I've done it now though – 10.13.4 – and like you said it's fixed the problems with Tradingview, both for Opera and Chrome. It's fast and smooth again, more so in Opera than Chrome I think.
I find it a bit strange that the acceleration was disabled when I never experienced a problem with anything until the new upgrade, but that's all done now. Opera's great and I'll continue to use it as my main browser.
Thanks again for all your help. Cheers.