This is because fullscreen mode allows the use of something called "DirectDraw Exclusive mode" which bypasses Windows for making calls to the GPU.

https://msdn.microsoft.com/en-us/library/windows/desktop/dd3...

I wonder if it would be feasible to use this to reduce input lag in an editor?

Yes, but you lose access to any user interface components you don't paint yourself, and you can eat some lag/flickering when switching out of the app as control is returned to the compositor. If you end up needing to show the Open File picker from the OS or pop up the Print dialog you'll need to exit exclusive mode.

It depends on the game for me, usually I prefer borderless windowed because I tend to alt-tab out a lot.

Me too. It's just so much quicker to alt-tab to my browser or Telegram or iTunes, even on my beefy rig it takes multiple seconds for my desktop to take control again.

In my experience the vast majority of games that I play don't suffer any perceptible ill effect from running in borderless fullscreen. If you're playing the game competitively it's another story, but for me running around shooting aliens in Destiny 2 or something I haven't noticed any degradation of my experience. There's of course the odd (usually older) game that doesn't support borderless fullscreen, but sometimes there are mods to support it.