Personally, screen real estate is the second most important metric as far as productivity goes, after RAM (I usually have a lot of browser tabs and terminals open). I usually use two 2k monitors and my Macbook Pro's retina screen when developing. I would definitely feel severely hampered by a single screen with 768p resolution.
I find the opposite, 2 or more screens tends to distract me more than it helps. I'm more productive on my 1366x768 13" screen as long as I have a decent window manager (i3, tmux) that can actually manage your windows. I'd do this all day if the ergonomics were better.
For me, no matter how fast I can switch windows, it's still faster just to glance at another monitor. And if I need to say, look at some example code in a browser, while having my own code open in an editor, as well as another terminal for reading output, it quickly gets very cramped at 1366x768.
