How to replicate:
1. Join a world with Vibrant Visuals on + with full screen
2. Test your performance (I have camera lag)
3. Click F11 to turn off full screen
4. Test your performance again (for me its much better now)
I also know of a case where performance dropped when playing with full screen off and increased with full screen on, but I cannot confirm this.
That’s pretty normal, when full screen is on, your computer will have to render more pixels, therefore causing FPS to drop (lag). You can probably get a similar result to using windowed mode by going into “vibrant visuals options” at the bottom of video settings, and turn the resolution scale to something like 66%, which basically runs it in a window and scales it up to full screen. It will make it look a lot worse but should reduce lag.