Although "smooth" is subjective, what you are describing as smooth may very well be a sluggish action or recoil cycle. I like the recoil cycle to be over as quickly as possibly because you can't start working on taking the next shot until the cycle is over.
In short, closed bolt lock time is the length of time that the bolt stays closed after the chambered cartridge is fired. The lighter the buffer, the shorter the lock time. Conversely, the heavier the buffer, the longer the lock time.
To little lock time along with too much bolt velocity will cause problems like stuck cases, accelerated parts wear and other reliability problems. Too much lock time and you'll see failures to cycle properly.
Messing with spring rate opens another can of worms. Although the spring rate has little to no effect on lock time, it can have a big effect on bolt velocity. You could be running your rifle in an over gassed state, but things may seem ok because the spring is reducing the velocity of the bolt. It could also turn a properly gassed rifle into one that won't run properly.
Man, I'm really not trying to be a jerk, but this whole train of thought that ARs are just tinker toys is very misleading. I'd suggest you get a book on the operation of the AR15. At least that way, when you start swapping parts in an expensive rifle, you'll have some clue as to what you're affecting. Swapping parts just because isn't a very smart thing to do.
With your recent purchase, it came with an adjustable gas block. I suggest that you start with that. The best fix for too much gas is to turn down the gas. Adding a heavier buffer to an over gassed rifle will make the rifle within spec, but it isn't going to reduce the amount of gas being run through the system.
