I have experienced a sudden drop in skyrim performance after installing hearthfire then dawnguard. I am currently running a vanilla version of the game (no mods at all).

Initially I tried turning down the graphics, then starting a new game, deleting saves, reinstalling the game from scratch, changing screen resolutions, removing power saving options on windows power plan. After all that I updated my graphics card driver after which the problem went away for a few days before returning exactly as before.

Since then I've tried a clean install of the graphics card drivers, have turned all power saving options off in the graphics control panel and have tried verifying the integrity of the game cache in steam.

I am having consistently unplayable low frame rate especially in battles and mouse/key response time lag. It was working flawlessly, then had issues, than was working flawlessly and now is having issues again.

Can anyone assist me?

If you are seeing purple fog, roll back to the graphics driver before the current one.
This fixed the graphics for me.
I have yet to learn why the scripts are no longer firing as swift as they used to before the last batch of MS updates.

No purple fog but thanks for the suggestion. I eventually managed to fix this by uninstalling avast antivirus