It's at least 5 minutes to save a cue. Can be shorter or longer depending on how many slides.
I am on 7.1.8 and I feel like it started taking longer after taking one of the recent updates, but has gotten progressively worse. Then as I've got more comfortable with the new version I've done more to the cues (transitions, move videos, etc), so I imagine that comes into play to an extent. But the extra stuff doesn't always come into play, I don't always add transitions for instance.
That wasn't a very definitive answer, but we can put it this way: I had noticed the time was definitely not as quick as saving a Word document and now it's just got to the point of saying something about it.