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.