Just an update for anybody that may stumble on this in the future, and also for the devs in case they choose to look into this later on.
I managed to figure it out and solve the issue. It was the "Use hardware acceleration when available" option. Turning it off removed all the stuttering issues. It also maybe explains why when the browser is not in focus the stuttering goes away (because hardware acceleration goes to the program in focus instead).
I don't know why it's like this though. Firefox uses hardware acceleration just fine, and my hardware should be more than sufficient to run a browser.
Anyway hopefully this will help shed some light. At least now I can finally start using this otherwise awesome browser.
So playing locally saved files (through the Windows media player AND through opera) does NOT cause lags. I can watch them without lagging. Right now playing YOUTUBE Videos works totally fine. I think Netflix is the problem. Netflix in Edge & Internet Explorer & Opera is causing laggs. Nothing else does.
So there is no Opera-specific problem, but one that affects all browsers equally in connection with Netflix.
Possibly, but this is only a speculation, these delays are due to Netflix checking the network traffic (periodically) for the possible use of VPNs . Since Netflix is not free, their customer service team should be interested in helping you with that issue.
The only driver I ever updated is BIOS and Graphics (today). Which drivers should I check aswell?
Especially with Windows 10 all drivers should be kept up to date, because the various operating system updates often entail far-reaching changes to which the drivers (for motherboard, sound card, printer, WLAN, Bluetooth devices, etc.) must be adapted. For security reasons, drivers that are up-to-date are also recommended.