Tearing is caused by lack of vsync -- if you turn that on under settings in graphics that will go away. As for slowdown I don't know -- there's no good reason for that. My guess would have to be that it's an illusion caused by the tearing. Honestly nothing changes when jumping or not jumping except the screen moves and some sprites are different. You should see the same tearing and so forth when running fast.
Hope that helps!
