Can you please answer all the relevant question in Posting Guidelines at the top of this forum? That will help us run down your issue quickly.
A few additional questions:
1) Is your CSS file odd in anyway, how large is it?
2) Do you have the Outline or Properties view open?
3) Are you experiencing slow downs only when selecting text – is typing and autocomplete fine or has that slowed down as well?
4) Can you try other CSS files, is it slow for all of them? Conversely, if you edit the *same* css file in 3.8.2 and 4.0, do you notice the speed difference?
5) How are you starting eclipse, what are the command line parameters?
Best,
Brian.