Speeding up video problem

Say I want to speed a video clip up to 3x speed. I apply the speed effect, set it to 3.0 and the video is now that fast.

But when I do this, the clip remains the same length on the time line, it's just now, two thirds of it are completely black. Is there a way to stop it from doing this? How do I export to MP4 without 2/3 of my video being a black screen?

Thank you.