Use a responsive editor (makes the most difference).

* Use a low-latency keyboard, if possible.

* Choose programs that add global keyboard hooks wisely.

* Turn off unnecessary “image enhancers” in you monitor.

* Enable stacking window manager in your OS (e.g. in Windows 7, 8).

TESTED EDITORS RESULTS, Average latency, ms

    IDEA-zl     11.9
    GVim	19.0
    Gedit	23.9
    Emacs	44.7
    Sublime     52.1
    Atom	90.0
    Netbeans	119.1
    Eclipse	133.6