The computer is powerful, nothing is overloaded, there are no limits, the Internet is in order, but scrolling on any pages is still jerky, and when new information starts to download from the site (for example, vk.com), the entire browser starts to slow down.
There are no extensions that can make websites slow down so much, VPN is not included, and the browser itself was installed only yesterday. In other browsers (for example Firefox), everything is completely fine, everything is smooth and without freezes. What is the problem?

My browser behaves badly. One open tab sometimes takes up 7 GB of memory. The browser slows down, the video goes to the full-screen window for 2-4 seconds. In games with an open browser, there is freezing and FPS sags. This was not the case with Mozilla's 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.