I may have a temporary solution.

1) Follow instructions here: http://pcgamingwiki.com/wiki/Divinity:_Original_Sin (Framerate Drops and Suttering section).

2) In-game, do Alt-Tab to any other application then back in game.

It works a bit for me - I have still big framerate drops during fight mode, but they are short and the game keeps a normal display in exploration mode.

I hope it helps.