To determine a "fast burn" head, we must have a "slow burn" head to compare it with. In the case of a SBC, we have the "conventional" '461 "camel humps" and the Vortec head which is considered "fast burn"(1996 and later head). You will notice on the Vortec a heart shaped chamber with a much more centered spark plug location and a good sized quench pad which extends up to and slightly between the valve seat area. This promotes good mixture motion as the piston comes up it "squishes" the mixture out of the dead area and back toward the spark plug, causing a remixing as it moves. This motion helps homogenize the fuel and air mix, which allows the flame to travel much faster as it burns. Also keeping the mix near the plug helps to reduce "end gasses" which are the unburned fuel and air particles trapped near the edges of the bore and top ring. Since it all burns faster we need to reduce the timing advance so we don't start burning too soon, since the piston needs to see peak cylinder pressure between 7-15 degrees AFTER TDC in order to accelerate the piston down the bore faster. Also since most of the combustion pressure is finished pushing the piston down after only 1/3 of the stroke, even thought it is still burning well after that, a faster burn means we use more of the combustion pressure to move the piston, and less of the burn goes into just heating the piston and block. If you can imagine this all happening in a low rpm street engine, think how it benifits a high rpm racing engine where the burn time is super critical. we can increase burn time with piston domes, or dishes to raise compression to the verge of detonation, but the burn time needed to push the piston goes up dramatically as engine speed goes up, so we need these benifits to keep combustion up with piston speed. A lot of other factors then start to come into play, rod ratios, and lobe seperation angles, and so on. Take every opportunity to look at the progressions of chamber designs made by the factories and note the changes they made all for the sake of combustion efficiency (burn speed/ mix motion), you will learn boatloads.
There are some really smart guys who post on here who could fry your brain with their vast knowledge on these things, but I hope I have been able to shed some light on this, along with the others who posted.
This is a good question.