Baldur's Gate II: Shadows of Amn/Underdark

The Underdark is home many mysterious monsters, including the drow. Luckily, you hardly have to deal with any of them to progress.

Svirfneblinsayswhat?

The first step is to get the light gem. You can explore all you want, but you're not getting out of underdark without visiting the silver dragon, and the only way in is with the light gem. The leader, Goldander Blackenrock hold this light gem at the local Svirfne... halfling village. . It can be pickpocketed, retrieved off his corpse, or you could complete a quest for him and get it nicely (yeah right).

Note: If you're willing to lose a party member over it, all quest items are transfered to the PC when a party member is kicked out. So a party member can grab the gem while the PC waits by the dragon's lair. The party member with the gem could be kicked, and the PC could instantly enter the lair.

Adalon the Observer

You have two speedrunning options when dealing with the Silver Dragon. You can talk to her, and she will turn you into a drow. This will allow you to bribe the drow guarding the exit to let you through. Or, you can attack her (or anything to make her hostile). This will open the door, but the drow guarding it will remain.

Notable Nasties

Invisibility will protect you from most of the dangers in Underdark, but beware the Saughin (fish people). They can see through invisibility, they can stun from range, and they can slow from melee. If you're lucky, you will only need to deal with three of them. If you're unlucky...

Should you survive long enough to get to the drow party near the exit without a disguise, they will make you wish you had one. A hidden rogue is constantly detecting illusions, which will not only break invisibility, but also some spell protections like blur and mirror image. The mage will begin to cast True Sight as soon as you are near. As a final obstacle, the exit is physically blocked by two high hitpoint and magic resistant skeletons. You can try to live long enough for them to move out of the way, or you can cast summon familiar over them, and have your familiar exit. If your PC and other party members are nearby, the whole group will exit. Don't save that constitution point though, because your enemies will have no qualms about following you through the door. This whole disaster can be skipped by just talking to the dragon and getting a disguise, but is it faster?

Overdark

Once on the surface, an elf will talk to you can put you through a bit of a cutscene. That is, if you let him. You can talk lock the specific elf who will initiate dialog and run away. This has an added benefit of allowing you to talk to Elhan before the Elf General. If you talk lock the general and initiate dialog with Elhan, he will try to have the Elf sages tell if you are speaking the truth. Since you never talked to General Sovalidaas, the Elf Sages never spawned. Thus, the dialog will break very early, and Elhan will act as if you participated through the whole thing. You won't get the abundance of stakes and holy water this way, but really, who needs 'em?