I'm having the exact same issue! No solution?
I have a fast (3.3 GHz) quad core processor with 12 GB RAM. Memory/CPU should not be an issue. I've updated Flash. I can switch over to Firefox and the same videos play perfectly fine. This is an Opera issue, not a system issue.

Also having memory leak issue

Opera 46.0 Stable 64 bit, Windows 7 Home Premium 64 bit, 12 GB RAM, Quad core 3.3 Ghz processor

I came across this Opera Forum thread just now while looking for a solution to my newly developed Opera 46 memory leak. I've been using the Opera browser for a few years now; I was previously a diehard Firefox user until that browser got slower and slower with every new version release. It would seem that the same thing has now happened to Opera and I'm so disappointed. I was simply trying to play Words with Friends on Facebook (a Flash application), and it kept freezing. When I opened my task manager I found that Opera was gobbling up memory non-stop at an increasing pace. As you can see above, I have plenty of memory and a fast processor so this should not be happening in normal use.

I closed the browser, cleaned my cache and re-started it. Everything was fine for a while until it did the exact same thing again, just playing Words With Friends on Facebook. (I tried it with and without AdBlocker + enabled and there was no difference).

Just putting in my 2 cents. I'll patiently wait for a solution, hopefully with an update. Like others who have described their issue on the forum, it seems that the issue occurs when using Flash player so it would seem there is a problem with Opera not playing nice with Adobe or visa-versa.