VSE rerendering whole video for just a simple effect sometimes I add very simple effects to videos (files are already in video format). For example I just add simple fade in / fade out effect in the beginning / end of the video like in the image below:



However this rerenders large portions of the video which haven't been edited at all hence adding these simple effects can be very time consuming. The frames between the gamma cross strips actually don't need to be rerendered. 
So my question: Is there an approach to skip these frames in order to save some time? (As I said the file is already in a video format, no image sequences availabe)

Thanks in advance !
You are rendering out the timeline, so everything in the timeline is rewritten, therefore you cannot just do the new bits.

However you could render the portions with effects as seperate clips then use an external app to join them to the original. A program like ffmpeg will do this appending function, but I don't recall if you can alter the start and end time of the un-effected clip in the middle. Anyway ffmpeg is a faster tool but at this point you may as well use Blender too render it all as a single piece.