Speed doesn't matter, stance is however you can best hold the gun.
How it works is, without a disconnecter to prevent firing with the trigger held back, when the action closes, the hammer drops.
The problem is, the hammer may fire the shell while the action is not quite locked.
This is HARD on guns, since the parts take a real beating. As the parts wear, you get into a situation where the gun is firing while unlocked at all, and the only thing holding the bolt closed is you hand on the pump handle.
This batters the gun even more.
The dirty little secret of slam firing a shotgun is, it's a "Hollywood thing" that looks impressive on a movie screen, but doesn't work in the real world.
The truth is, it's something to do on a firing range for fun and to impress people who know little about guns.
It's the shotgun equivalent of fanning a single action revolver, or pointing a high capacity auto at a target and yanking the trigger as fast as you can.......looks impressive, hits nothing.
It's hard on the gun, and can be dangerous.
In fact, it's almost impossible to actually hit a target while slam firing which is why no real combat shotgunner will use it.
In the movies, the shotgunner, (often with a pistol grip short gun) points the gun in the general direction of a target, slam fires the gun, and a 20 foot wide glass window is blown out and the target is blown apart by at least 30 pellets of 00 buckshot.
In the real world a shotgun only works when it's aimed at the target, either by using sights, or by "pointing" the gun, which is how bird shooters and good combat shooters do it.
For this to work, the butt stock has to be in your shoulder and you have to be looking down the barrel.
When done this way, the power of the shotgun usually means it's one shot per customer to do the job.
Slam firing wastes ammo the low magazine capacity of the shotgun really can't spare, and DOES NOT produce hits on target in real life.
In other words, if you're looking for a range toy to play with, slam fire away.
If you're thinking of using it in a real defense situation, likely you're going to get killed by an opponent who'll be actually aiming his gun and won't miss, while you scatter shot all over the place without effect, except possible innocent neighbors or bystanders.