Scrolling slows down but only in some areas, like the restored outdoors of the Temple of Amaunator and the area below the Cult. Every time you press a Numpad key to scroll it takes 10 seconds of hard drive use before anything happens! This was not evident in the last playthrough, where I had no Bioware NPCs in my party at all. So it should be connected to the presence of NPCs in the party.
Currently I have found two areas affected, but I have only played through bits of the game.
AR0204 The Forgotten Believers -- Temple City (Unseeing Eye Plot) AR0007
AR1400 Shadow Temple Land (Restored) AR0125
Approach, delete one NPC at a time and see if the problem then goes away. No it does not.
Imoen has a Special script 2 of "Default.bcs", this is not found in the IWD2 scripts. I created a new script, "Default.bcs" but this did not fix the problem.
One approach that does seem to work, create your own versions of the Bioware NPCs:
Create a new PC of the desired class and race.
Open the relevant Chr file in Near Infinity and make the necessary adjustments to the file.
Save the file.
Necessary adjustments give your NPC the dialog, script and identity of the real person. Relevant offsets seem to be:
Name 22c h
Apparent name 230 h
String 3cc h
Override script 498 h
Special script 2 4a0 h
Combat script 4a8 h
Special script 3 4b0 h
Movement script 4b8 h
Death variable 5b8 h
Dialog 84a h
New values come from the Chr files of the official NPCs.
This way you can customize the NPC's class(es) to your taste.
It may be that an NPC joins but no Chr file is generated, so you have the NPC being read from a Cre file instead. There is no way to verify this theory because the game crashes when you try to save…
In IWG2 NPCs are limited to 76 points on creation, plus any racial bonuses. Even after allowing for the 4 points of ability increases that should occur during the game, most NPCs end up with fewer points than they had in BG2. Halfling Mazzy is down 5 points in Strength and she is a Paladin!
Using Near Infinity to alter NPC variables like scripts also gives an oppurtunity to alter Ability Scores. Normally the idea here would be to apply the knife and force each character to have no more than 76, 78 or 80 points. However this may obscure the identity of each character. For example, the temptation would be to lower the Charisma of Imoen to save on ability points, but she is quite charismatic and this would be reflected in what she says and does.
BG2 did not enforce the pen and paper rules about maximum spell level being based on class level and Intelligence/Wisdom. A few spellcasters have bizarrely low ability scores as a consequence. Serious spellcasters in IWD2 need a minimum score of 15 in the spell casting ability concerned to stay on track for attaining maximum spell levels.
Certain races end up with two or four more ability points than others, +4: Aasimar and Drow Elf, +2: Tiefling.
Other races end up with two points less than the default: Gray Dwarf, Deep Gnome and Half-Orc, but none of the Bioware NPCs are from any of these races.