why login screen has a 2-second delay with external monitor?
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=