Here are some of the areas I think that could do with a reduction in NPCs and getting rid of summons:
Summons1) Osgilliath - the gondor healers and/or wizards all have summon creature # which summons those TS dragons. The mordor NPC don't have any summons. Won't help much with lag but removing these summons will help those new lower level players of the mod trying to cross osgilliath under hide/invis as these dragons with TS spot you and cause the new AI of the Gondor npcs to kick off.
2) Barad Dur - all clerics and wizards from the black gate to final BD map all have summons. The clerics summon something pretty useless, while the wizards summon both a familier and an epic dragon knight. Most noticable consequences is heavy overcrowding, heavy lag and npc target interference in BD gate and BD boss areas.
3) Rivendell - clerics all summon ruby golum, other npcs summon familiers and epic dragon knight (see NPCs Rivendell below).
4) Lorien, Mirkwood including King Thranduil lair - all druid NPCs spawn elemental swarm. Consequence of heavy overcrowding, lag and npc target interference in these areas.
5) Mount Mindollum - I don't know about anyone else but I think the summons the Orge mages spawn (snake things) make this place not very practical to new players to level. They throw cold bolt attacks which do up to 40 damage, usually spawn in 4s so unless your a high constitution build then your hp disappears pretty quick.
NPCs1) Rivendell - reduce number of NPCs and remove/reduce power of rivendell fairies. In combination with above summons, all Rivendell maps become very overcrowded, laggy with npc target interference.
2) Dale - hard to know if this is a spawn bug or general problem with too many npcs, but this map is impossible. Map gets quickly overcrowded with very high level NPCs which results in it becoming a run-through map. Bard boss can be killed before main spawns are hit but after this the map becomes unplayable. Remove perma TS/blindsight from Clerics/Mages - they are clerics and can cast the spell
3) Lonely mountain palace - same problem as Dale.
4) Dunland - the spawn chaining here causes overcrowding.