Bad exports from sped up videos

When I speed up videos, it gets very choppy/pixellated. I don't know how to describe it, but it didn't happen on the first video I made so I'm confused as to why it's happening now. The video was around 40mins and sped up to 5 minutes. The original video didn't have anything wrong with it (i recording with obs, don't know whether that's relevant but..just in case). The link is what the video ends up looking like. I exported it twice, once as in & out and once as contents, and it did the same thing both times. 

 

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