The porting was originated on International Skeet and trap originally to take the pressure off the wad near the end of the barrel, reducing the chance the wad would move into the shot cloud just off the muzzle, and "blow" the pattern. The Russians also came up with the "Tula" choke for much the same purpose: dense, uniform patters.
However, there has been much experimenting with porting for holding down the muzzle, especially for he first shot on a double OU.
Regarding placement of the holes, they should be near the muzze from the standpoint of Physics. I want them in the choke, not the barrel, no matter how sexy the "Buick Portholes" look down the side of the barrel.
The vent chokes are very effective, and make negligible shot speed reduction, working mostly on the residual gas pressure after the shot exits.