If you want my best guess at the possible problems, it may well have to do with directing OpenMW to a modded installation. All the mods that are in the install where you directed OpenMW are there for OpenMW as well, and if you've got any poorly optimized graphics mods, those will definitely cause a hit. It's quite possible that some of them may cause more of a hit in OpenMW than in vanilla due to something wonky that the vanilla engine does. Wouldn't be the first time that problems only really became visible on OpenMW despite having been there the whole time. There's also the possibility that the problem is related to the collision engine. I know that in the past, a bunch of people, myself included, had FPS issues in places like Sadrith Mora due to NPCs walking into objects continuously and thereby generating a bunch of excess work for the collision engine. That could also be related.
As far as stability goes, what makes OpenMW so much more preferable is less the FPS, and more that it doesn't crash a bunch, and long-term use of a single save file is much less likely to run into corruption issues. You can also more easily swap mods in and out, though I think that still isn't recommended, and you don't have the hard limit of 255 plugins. Basically, the vanilla engine is buggy and often unreliable, though when it works, it works well, where OpenMW, to steal a quote, "just works," but it still isn't necessarily always up to the standard of when the vanilla engine is running optimally quite yet. Thing is, the vanilla engine rarely in my experience runs optimally for very long, and when it starts to go downhill, oh boy does it go downhill. Generally speaking, once you get a functioning OpenMW install, it will stay that way. That wasn't always true with the vanilla engine.
Hope this helps!