Windowed behavior differs even between various Win 10 versions. But generally can be said that in Win 10, it usually adds a bit of lag, because Win 10 force triple buffered vsync when running windowed. Though at least in the latest version, it's a weird kind of vsync, because while it removes tearing, it doesn't limit your framerate to your refresh rate like vsync normally does if you're running the game with vsync off. So it gets a bit more complicated then.
I'm on the latest Win 10 version and I do run rF2 in borderless, like I said, and I use vsync (the Video one in rF2 settings), and using the frame limiter trick (limit your framerate to around .005 to .010 of your *exact* refresh rate via RTSS) and also the ultra low latency mode in nVidia drivers, I get lag that is quite acceptable for me, even though I consider myself being quite sensitive to it. Another option for me would be to run the game at double the refresh rate with vsync off, that would make the lag even less, but my CPU is too weak to be able to sustain 120 fps in rF2 at all times, not enough single-threaded power. And running vsync off with framerate limiter set to anything below double the refresh rate is not as smooth, but more importantly, it also causes random microstutter for me, so not really a better option.