I maintain a decent space cushion, and if people use it to merge smoothly then so be it. I always let people in if they’re willing to merge properly in a way that doesn’t cause me to slow down.
If I see a dickfor like zwarbyt 
who’s afraid of coming in last come flying down the ending lane intending to swerve in at the last second causing everyone behind him to hit the brakes, I move over and block them. It’s those assholes that cause the slowdown in the first place because everyone has to slow down to let him in, so I’m doing my part to help.
It’s not like I’m realistically going to get where I’m going any more than a few seconds later than those who are too hopped up on goofballs to merge safely and properly.