So you've never used a good one is what you are telling me.
A good two stage trigger allows you to maintain nice positive sere engagement surfaces that are large enough to provide a high degree of safety from undesired discharge, while at the same time rendering a light to very light sere release on the 2nd stage due to the design of the trigger mechanism.
For example the Geissele 2 stage trigger depending on variant will usually offer a 2 pound first stage that takes up the majority of the actual sere travel, due to the high degree of care in manufacturing this first stage is smooth as glass. Upon reaching the end of the first stage, the trigger by design will not release the sere until an additional spring is overcome that is not loaded until the initial travel has been taken out. This second stage is nominally an additional 2 pounds or lighter, and is a crisp consistent break. They also offer lighter 2 stage units, typically with the 2nd stage being lighter down to around 1.5 pounds. Because most of the sere travel has occurred at this point the 2nd stage trigger travel is very short, just like a single stage trigger.
What the shooter feels or perceives is a trigger that smoothly takes up to where the sere will break, and a very light crisp trigger once you take up that slack and get to the "wall" of the 2nd stage. Because of this the trigger will feel much lighter than it actually is, and in reality it might as well be from a marksmanship standpoint since you are not fighting with the total trigger weight all at once to overcome the sere travel needed to break the shot. So while a 4.5lb single stage trigger will feel oppressively heavy, a 4.5lb 2 stage can feel phenomenal when set up correctly because what you perceive and what you really get is a trigger that breaks with the weight of the take-up which you won't even notice and then a crisp let off with another 2 pounds of pressure. So it feels like a crisp 2 pound trigger, makes the rifle perform like it has a 2 pound trigger, but has a safety margin like a 4.5lb trigger.
In all honesty at this point I shoot 2 stage triggers better than I do single stage triggers, I find the first stage of take up and super predictable "wall" of the second stage allow me to consistently break shots when I want to. Anyone who has shot very light single stage triggers has experienced the hated shot where you are refining your sight picture and the gun goes bang while you were trying to start building pressure on the trigger.
When shooting quickly, there is no real performance difference outside of gun games between the two, just press through the trigger.
