I guess the title makes it quite obvious this is a rather specific query!
I was curious why a particular combination of the above can cause really bad performance problems, and if anyone has any ideas. I've been faffing about with the road record in order to correctly add two new provinces to my game, Elsweyr Anequina and Valenwood Improved; prior to that, NPCs of a more wandery bent weren't able to travel, and now they are, so all is well there (for the record--which was an unintentional pun, honest!--I've started off with Arthmoor's excellent NRB road-generation grid and manually mapped the new provinces myself; which seems to have succeeded in spite of me forgetting how to delete the existing pathgrids en masse.)
All has been well, for the most part... unless I'm somewhere in the south, pretty much anywhere below Bravil but especially Leyawiin, or Haven in Valenwood, if I look towards Corinthe (near the south of Elsweyr) I get appalling framerate drops, typically under 10 fps with awful lag. Normally I'd expect at least 40 in outdoor areas, usually much higher.
(It's possible Corinthe itself could be the culprit since open cities aren't things Oblivion does well, but in this case it's so distant from the areas affected that it seems unlikely.)
The weird thing is, I only get it when I have Streamline active: it's fine otherwise, but no amount of faffing with its settings seemed to rectify the problem. Of course I don't want to disable Streamline as it's pretty much essential when using Better Cities unless I want my graphics permanently on minimum. Also for the record, I additionally have the Oblivion Stutter Remover installed, but I've told it not to monitor the min/max fps.
I have found a kind-of solution which is to reduce uGridDistantCount from its default of 25 to 15: this has little visual impact for me as I have the weather set up to always fog at a maximum of 40,000 units: that was influenced by Morrowind and intended to make the world look bigger (something I found wasn't the case in Morrowind if I used MGE and turned off fog!) Because of that, having 102,400 units worth of landscape being rendered is a bit of a waste as all I see are silhouettes of distant trees and objects: I don't want to nuke them all but it's rather inefficient as it is, so 15 seems like a good compromise. It has mostly done the trick, that <10 fps and lots of lag is now about 20 fps and some lag in the worst-affected areas, but I'm still curious as to exactly why it's happening: even with a solution, knowing the reason would still be nice!
The other benefit is that it's consuming less memory so I can now go for a startling 30 minutes or so between crashes, usually, whereas it was down to under 10 which was becoming almost unplayable. I guess I just have too much stuff loaded up, which isn't a good idea with Oblivion's extremely fragile memory management, but it'd be nice if this could also be mitigated somehow; sadly, Oblivion seems to be immune to the 4GB patch and won't use more than 2GB. But I guess this is getting off the point a bit...