this issue has been around since day 1, with or without mods. when leaving an interior to an exterior cell, the otherwise smooth game fps drops to a slideshow (less than 10-12 fps) at random. saving, exiting and restating gets it back to normal..but it's only a matter of time before it hits again.

is there any workaround for that?

Answer for that is a straight and simple no. It's a bug in Skyrim which effects lots of people (including me). It can be somewhat reduced for some by setting your shadow settings from Ultra to High.

Plus there is a faster way to get a smooth exterior:
1) Before opening Skyrim, just make an empty text file (txt) and leave it open.
2) Start Skyrim and play.
3) When the FPS drop hits, save the game and use the Alt/Tab keyboard combination and select the empty text file.
4) You will now be back at your desktop with the empty text file.
5) Now use Alt/Tab again and select Skyrim. You see a black screen now. Select Skyrim again with Alt/tab.
6) Now your back in Skyrim with smooth play.
7) When FPS drops again, repeat these steps.

This all sound bit complex but it only takes less then 10 seconds to do.