As in Title - I think I'm not the only one with this issue:
Outside - everything fine. Going don to the house or a dungeon and FPS drops to about 10-15.

I don't give a **** about fancy graphics. I just want to play ANYHOW.

Checked Skyrim Nexus for perfomance bost twaks, same with Google overall - no results.

My rig is nothing special (between minimal and recomended settings) and it's NOT a hardware problem - as mentioned above, it handles the outside areas smothly.

Help. Someone.

Post your hardware specs and settings you play at.

This.

Also, do you use ENB?

Anti-aliasing settings can cause this behavior because there are generally more surfaces that receive AA in interiors than exteriors. If you are using AA, try turning it off completely or only using FXAA. One area you can test this to an extreme is Frostflow Lighthouse.

have you tried play in lower graphic settings?

Intel Core i3 2.10 GHz
Geforce GT 520M
RAM 3 GB
W7

Using no mods at all, already lowest settings.

Disabling shadows at all helped a bit (A BIT).

Goes like this in order of most impacting to least indoors(for the most part)......AA(non FXAA), Shadows(they move when fire is near therefore indoors like in Dragonsreach tons of fire), Particle Effects like the indoor fog etc. There is an addon that removes indoor fog you could try that also lowering shadows and disabling AA obv will help like people said above.
http://skyrim.nexusmods.com/downloads/file.php?id=9571

The problem you have is the graphics card on your laptop. The 520M is on par with a 2004/2005 Nvidia 6800GT desktop card in terms of relative graphics horsepower. Your CPU isn't helping much either. The new i3's are great, but the first generation ones weren't the best for gaming, unless you overclocked them a lot.

Outdoor stuff in Skyrim does generally run a bit better than indoors, but even my system struggles with some indoor stuff (Thieves Guild springs to mind) and I've got a GTX 560Ti which runs most things without a hitch.

Try that fog mod Tedman and disable water reflects and try the Cistern again.