oddly enough i get the opposite effect, a 5-6fps increase when playing uboat add-on, but thats probably just a quirk of my crappy laptop integrated graphics.
Anyway, it could be because the IX sub is a stupidly detailed model and has quite a bunch more polys than the fleet boats, plus i think some changes were made to the environment/ocean with the add on which may be hitting your framerate.
Theres probably not much you can do, just go through the graphics menu and keep turning stuff off untill its playable again, personally i find volumetric fog and full screen glare have the highest impact of framerates.
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