There are so many Balmora expansion mods, I don't know which one that is. Anyway, I don't think I've ever used one. With the graphics mods I'm using, along with Starfire's NPC Additions and Children of Morrowind my FPS is already low enough in Balmora.
Its this one - the mother of all Balmora expansions:
http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=373
Plus Slartibartfast has a texture fix to it as well:
http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=4324
Here is the Lite addition by Slartibartfast to help with fps:
http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=4382
And I havent even looked into the recent update yet:
http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=8647
I wish I knew how to make these links into shortened phrases, never learned how to do it sorry....