You have two options: You can adjust your own speed slider (before export) to the slowest setting that still runs it nicely on your machine (before it starts to slow down too much), and then export as before. That should make it slower for everyone. Or you can uncheck the 'Lock scenario' checkbox in the Export dialogue in Greenfoot. This will give your scenario in the Gallery the speed slider and allow users to adjust the speed themselves.

Even with the speed slider this still seems either very fast or very jerky. Making everything move in much smaller steps will make the whole game appear far smoother.

This does not go to the second level for me. If I clear the first screen, it crashes. The reason is incorrect capitalisation in the name of an image file. You image file names end with a suffix in capitals (e.g. "EndLevelOne.JPG"), but in your source code you write the suffix in lowercase ("EndLevelOne.jpg"). This works on Windows (because the Windows file system is not case sensitive) but not in other systems, such as inside applets. This is why it fails in the Gallery. The fix: make sure your image file names in your source code EXACTLY match the names in your file system.