Can somebody please tell why, in the open world of skyrim, in some areas there is less lag than areas that are indoors and less complicated to process.

I went into a fort just moments ago and the fps was insanely bad, it was unbelievably laggy, so i cleared the fort out, completed my quest and exited the fort. Upon exiting there was a slight stutter as there is with every exiting and entering, but shortly after that there was smooth gameplay for a decent amount of time.
So im wondering how can it be more laggy in a indoor environment than outside in the whole world of skyrim :$
And its not just forts its some caves as well.

Please sort it out.