Environment variable OPENMW_PHYSICS_FPS=15 should give you a pretty nice boost. There are also some settings for reducing the memory footprint of navmeshes that you should investigate as well. You might even want to try using a build from before navmeshes were merged to see if that improves things. Disabling all preloading could make a difference, but that comes with its own big downsides (such as temp freezing at cell borders).