Hello. Finally it seems to happen to Slack and Rythmbox too. So I don't really know if is only an Opera issue or a system one. I'll try to reinstall MATE or the notifications library or whatever is involved in this thing.
Maybe the built in adblock then? Try disabling it for a test.
For the record and hardware wise, I am on an ssd. on an old intel core 2 duo and I have opera's cache in ram.
I do not know if it is related, but firefox, having better html5 support than opera, plays the youtube videos in h264 while opera in webm.
If this support regards audio files as well, the problem becomes aac (which is supported by firefox and not by opera) vs ogg.
As for what you can do about it, read the post on my signature.