why login screen has a 2-second delay with external monitor? On a brand new mid-2013 MacBook Air, with an external monitor ASUS PB278Q 27" 5ms WQHD (2560 x 1440), connected through a miniDP-to-DP cable, every time the computer gets back from the monitor sleep, the login screen has a delay of several seconds, prior to the typing or the updates being reflected.

This generally happens about a second or two after MBA's own monitor gets online, and the unresponsiveness continues for a couple of seconds, until the external monitor appears online.

Why do I have such a big screen delay on login, and how do I deal with it? 

My conjecture of the situation is that when my Mac instructs the monitor that it is no longer needed, it enters some kind of power saving mode which seemingly makes it unintentionally treated as a disconnect from the Mac, and then when the Mac wakes the monitors back up, the external monitor is made to appear as a new device, and takes some extra time for configuration / adjustments.  In any event, I'm convinced this should not be happening, but who exactly is to blame, and is there any way to deal with this?
A new conjecture is that this is due to a non-compliant mini-DisplayPort to DisplayPort cable that is causing the problem.

http://support.apple.com/kb/TS3533


In certain circumstances you may notice a black screen after waking your computer from sleep. The computer may also appear to be unresponsive when the black screen appears. 



Certain cables advertised as being VESA 1.1a compliant may contain a construction issue that may result in the DisplayPort power pins at either end of the cable shorting together. VESA 1.1a compliant cables must isolate the DisplayPort power pins at either end of the cable.


Supposedly, this only affects resolutions above 1920x1200, e.g. of the popular resolutions, only 2560x1440 and above are affected.

The StarTech mDP to DP cables are not VESA compliant, and very few mDP to DP cables are actually compliant.

There's a web-site with a list of compliant cables: 

http://www.displayport.org/products-database/?products_category[]=cables-connectors&amp;products_manufacturer=