30 to 15fps is not what I would call a big frame rate drop. That to me just sounds normal, like your hardware just can't keep up with sudden average features being rendered / loaded. I've had the game drop from a capped 144fps to 10 - 15 simply because I walked by 50 or so full grown trees clustered together during the day time... had to lower my graphics settings from Ultra+ to just Ultra which solved it for me (I wouldn't have thought the change in fps would be so drastic, but it is). And I didn't always have a top spec PC like I do now. There was a time when running this game on medium settings was the best I could do, and that was back when the minimum specs standards were set way lower. So I've experienced both sides.
But my point is, a few tweaks in the graphics settings is usually all you need to solve the fps issues (so long as you're within minimum hardware specs of course). Just keep playing around with the settings and you'll eventually find a sweat spot. If possible, try to aim for at least 45fps average, so that when big things get rendered or loaded, the fps drop isn't so substantial. The game might look worse in the end, but in my experience, smooth fps matters a lot more. A good start would be disabling shadows and reflections completely. Keep the resolution at or under 1080p. Everything else is a matter of experimenting to see what average quality setting works best for you. Don't just rely on that thing I forget what it's called, but it auto adjusts your settings as you play... it's a nice feature to have enabled for those with low end hardware, but it's best to disable it while you're trying to figure out optimal settings, then re-enable that auto tuning feature thing so that things run much more fluently and not have sudden random jumps in quality changes with fps all over the place.