Okay I've been through this a lot now. This is driving me crazy...
I have the weird problem with FPS losses whenever I exit building or anything like that, FPS losses.
Yes, I have 64 bit windows, 4 gb of RAM, gt 550 ti. What can be the source of this lagg? It happens when I fast travel and exit buildings, pretty much thats the main thing, and I searched all over the internet for the help, but nothing helped me yet...

Seems like your GPU gets hard time loading the surroundings. Do you have the high res patch or any graphical mods installed? Good idea is to renew your drivers with more recent ones or some older but more stable version.

Try turning off the Autosave features, that can slow down cell loads.

Also, see this:

https://www.youtube.com/watch?v=iR4eaaqZwGY