Clone / extend screen causes 20 FPS in full screen, why is that?

Have 3 screens and 1 TV that all hang on a graphics card (1080ti). As soon as I clone only one screen with another or an extension, the FPS always drop to exactly 20 but only in full screen (mostly also borderless window mode).

The problem is that I like to play on the PC with the TV from the couch and very few games offer a mode to display the game on another display device.

It doesn't matter which screen that is, it happens with every combination, and it doesn't matter whether HDMI or DP. In addition, it doesn't matter whether there are 2, 3 or 4 screens, the problem occurs with 2 screens, so it's not the number either.

Problem occurs not only with games in full screen mode, but also with video players such as Netflix or Youtube.

The drivers are all up-to-date, and various graphics drivers have already been tried out.

You can only have one main screen under Windows, all others are automatically operated at 20-30 fps. So if you clone the screens now, everything will be reduced to a minimum.

But the problem is only in full screen.

If the game is in windowed mode, it will run at normal FPS

Yes, it's up to Windows. The main screen is the main screen as soon as a full-screen application starts. So for example with you-tube