tldr when you use windowed borderless, DWM is handling your game being drawn on your screen, which is the desktop compositor. it waits for every other application (even if there are none) before pushing a frame, and there's always a bit of overhead as it re-renders the frame before displaying it. this causes inherent input lag, but the amount varies from system to system, and for some combinations of people/systems, it isn't noticeable.
in theory you should not have a lower framerate, in reality you might slightly because the GPU prioritizes your application slightly less since it's also rendering DWM's re-render while still trying to render the game. the input lag is the noticeable part though, and unfortunately it's kind of hard to measure.