Baldur's Gate II: Shadows of Amn/Brynnlaw

Welcome to Brynnlaw, home of pirates an prostitutes. A good speedrun will skip most of this area.

Entry to Spellhold

There are several ways to get into spellhold. The fastest and easiest seems to be to talk your way in.

Desharik

Desharik is the leader of the pirates on the island. To meet him you need to either pay 300 gold to the door guard, or complete some extensive quest and get a name drop. Since you should have infinite gold, the choice is clear. Next, you need to talk to Desharik. A charisma of 15 or more is needed to convince him that you're crazy enough to need to be sent to spellhold. Failing that, having either Yoshimo (since he's working for the same guy as Desharik) or Minsc (since he's... crazy) will also allow a quick entry into Spellhold.

The Wardstone

Alternately, a wardstone can be taken from a local Cowled Wizard, either by talk lock pickpocketing or off of his corpse. This is much slower, as you then need to walk all the way to Spellhold rather than being teleported in.

Spellhold

Upon entering, you are allowed to explore, before eventually being knocked out and taken prisoner, again, by Irenicus. That is, unless you're better at exploring than the designers intended.

To Underdark

There are two locked doors that must be wall jumped to get to the underdark and skip a huge portion of the game. The first is shortly before you meet Irenicus again, to the west of the cells. This will bring you downstairs. In the southern part of this floor, there is a hidden door with a portal behind it. Jump this wall and walk towards the portal. Note that the portal is not actually what sends you to Underdark; there is a floor trigger before the portal. This means you don't need to worry about having every party member nearby.