Some reasons why the Save World feature is greyed out are: (a) no changes to the world or to objects in the world were manually performed (b) one or more act cycles had been executed (c) the project is not compiled Of note, for (a), setting the image of a class is not manually performing a change to an object of the world; but, adding an object into the world, right-clicking on it and selecting 'setImage' to set the image of that actor would be actions that can be saved by the Save World function. For (b), comment out 'Greenfoot.start()', if you are using it, so no act cycles will execute. You said it was completely compiled, so no comments on (c) here. If none of these are culprit, you may want to specify which version of greenfoot you are using before contacting the support team, if this seriously could be a bug within greenfoot itself (check the bug log first to be see if has previously been reported).

Do you get any terminal messages? I had something similar, I used this on my images and it seemed to clear up the crashes / slow downs:

From what davemib123 pointed out, I don't seem to have any terminal errors but from what danpost pointed out maybe the issue which was '(b) one or more act cycles had been executed'. Could you explain in further detail what this exactly is because I am confused and it could be the problem at hand.

If any act cycles are run after recompiling or resetting (either you press the 'Act' or 'Run' button or you allow a 'Greenfoot.start();' command to execute, which automatically starts executing act cycles), then you will not be able to save the world. In other word, reset the project and do not allow any act cycles to execute; proceed to manually make what changes you need to do and then you should be able to save the world.