Gough Thomas (a well known and respected expert of 30 years ago for UK Shooting Times) believed that when correctly used, the true self opening side by side was the fastest without a loader. He used a Henry Atkin "Spring Opener" (Beesley/Purdey type self opening action).
The way it is used is this (for a right hander)
After firing, the right hand alone brings the gun down, whilst the left hand goes to belt/pocket for fresh cartridges. As the gun nears fully down position, the right thumb operates the opening lever. The gun self opens, ejecting the spent cases as it does so - meanwhile, the left hand has collected new cartridges and inserts these in the open breech. The left hand now goes to the barrels and closes the gun.
Done by a skilled, practised individual it is a very fast and very elegant technique, but it is quite hard to do well and needs quite strong hands!
