I'm noticing a severe FPS drop (below 10 FPS)only in the Imperial City (everywhere else it's ~30ish) when using these mods (and in this load order)

00 Oblivion.esm
01 Jog_X_Mod.esm
02 All Natural Base.esm [Version 0.9.9]
03 Francesco's Leveled Creatures-Items Mod.esm
04 Francesco's Optional New Items Add-On.esm
05 Francesco's Optional New Creatures Add-On.esm
06 Cobl Main.esm [Version 1.72]
07 Unofficial Oblivion Patch.esp [Version 3.2.6]
08 UOP Vampire Aging & Face Fix.esp [Version 1.0.0]
09 Oblivion Citadel Door Fix.esp
0A All Natural - Real Lights.esp [Version 0.9.9]
0B All Natural.esp [Version 0.9.9]
0C All Natural - EW + NW + AWS.esp [Version Final]
0D WindowLightingSystem.esp
0E Adonnays Ranger Coats.esp
0F Harvest[Containers].esp
10 Harvest[Containers] - Player Home Add-on.esp
11 Harvest[Containers] - Flat-Top Barrels Add-on.esp
12 Harvest[Containers] - Havok Crates Add-on.esp
13 MaleBodyReplacerV4.esp
14 Thieves Arsenal.esp
15 Francesco's Optional Files.esp
16 Cobl Glue.esp [Version 1.72]
++ Cobl Tweaks.esp [Version 1.44]
17 T.I.E4MODS.esp [Version 0.41]
18 thievery.esp
19 HoarfrostCastle.esp
1A Harvest [Flora].esp [Version 3.0.0]
1B Salmo the Baker, Cobl.esp [Version 3.08]
1C StealthOverhaul.esp
1D RenGuardOverhaul.esp
1E Francesco's Slower skills x1.5.esp
1F Francesco's 10 days respawn time - 1-20 day lenght rescale.esp
20 [GFX]_Initial_Glow-all.esp
21 [GFX]_Initial_Glow-self.esp
22 [GFX]_Initial_Glow-creatures.esp
** All Natural - Indoor Weather Filter For Mods.esp [Version 0.9.9]
23 Bashed Patch, 0.esp
24 Duke Patricks - Combat Archery.esp [Version 2.3]

But if I deactivate them all, there's no drop. I'll probably end up having to go and deactivate one at a time, but I though that someone might have some more refined insight as to which mod it might be. Thanks!

20 [GFX]_Initial_Glow-all.esp includes both self and creatures ESP (if my memory serves me well).

I am not sure, but I guess that Real Lights could be the one causing the drops. It is optimized, but in IC there are a whole lot of light sources.

Bang on sir, thanks very much!

You should try using a less complex version of AWLS too, if you need more FPS.