Here's the story:
A couple of months ago, I got a nostalgia attack and went back to an old minecraft version: 1.4.2. I tested with mob farm mechanics and after some time: This popped up in my world!

Note: If you import it in your world, make sure you have few other spawning spaces for hostile mobs to spawn except in the Farm.



Here is the changelog:
Version 1.0: (Oct. 2013)
52,000 items per hour. It lagged so horribly I couldn't bear even looking at the farm itself.

Version 2.0:(Nov. 2013)
83,400 items per hour.There seems to be around 1200 entities when I finish a 1 minute efficiency test. Lagged my PC and I crashed a couple of times.

Version 2.1(Nov. 2013)
88,000 items per hour. Found a problem in the water streams/ canals: Items get stuck in them, some items don't even reach the actual water stream. Fixed!

Version 2.2 (Mar. 2014)
I took a break from making mob farms. And I returned. Fixed more item stream issues. (Items were stuck in the sign.)

Version 2.3 (Apr. 2014)
93,000 items per hour. 4 spawning floors were cut. The farm now had around 18,000 m^2 of spawning space.

Version 2.4( Jun. 2014)
94,000 items per hour. Optimization of fence fall trap. Occasionally, mobs used to survive the fall, which proceeded to lag my PC and fill up the mob cap.

CURRENT VERSION (Will not be updated)
125,000 items per hour MAX. Reduced some more spawn space.

MCEDIT schematic now available! Enjoy! (Remember to use versions prior to 1.4.4)