It is true that many objects will slow it down too. I wasn't quite clear enough about that. But, I was trying to explain why some scenarios seem to run smoothly with many objects where other scenarios don't. You can't directly use the number of objects as a measure of how fast a scenario will run. There are many other factors influencing the performance that might not be obvious. Some examples of this are: objects rotating, objects moving, objects added, objects removed, the distribution of objects, the use of intersection tests. For realm of battle, adding fog of war will make it much more expensive to do collision checks between the other objects, because the fog objects will also be part of the structure that will have to be examined. Greenfoot's collision detecting mechanism could be improved in many ways, but currently it is not high on our priority list, even though it is a very interesting problem.

How fast it runs also depends a lot on what the objects are doing. If they are just moving without interacting with the other objects it will run faster than if you for instance check for collisions with other objects.

I found out what the problem was. I did try to reproduce the problem by adding a space at the end of the title, but that worked fine. However, adding a space at the beginning of the title made a new scenario. We will get this fixed. Thanks for the help with tracking down this bug!

The problem is the name of the "Images" directory in your scenario. It should be "images" with lower case, not "Images".

This is also a problem with the case of filenames. The file 'fondoJuego.JPG' has the wrong case. It should be: 'fondoJuego.jpg'.

This is a problem with case sensitivity of the filenames. The filenames you use in your source code doesn't match the actual filenames when case is considered. Rename your files so that they all are in lower case. The offending files are: pelea#.GIF, policia#.GIF and sangre#.GIF. The extension should be "gif" not "GIF".