The Stargate Command dialing computer allows for the dialing of a gate without a corresponding Dial Home Device (DHD). While the custom dialing system has certain advantages, one of it's biggest drawbacks is that it takes considerably longer to dial than a DHD, due to having to physically rotate the ring of symbols.
In the episode "Serpent's Song", Samantha Carter modifies the dialing computer program to be able to dial much faster than it used to. In fact, with the modified program it is capable of dialing more quickly than someone using a DHD.
After this episode though, the dialing computer seems to be reset to its old, slow mode of dialing. For example, in the opening of "The Light" it takes at least a minute for the gate to be dialed. Why doesn't the SGC always fast dial after gaining the ability to do so?