Tough question you have there..Since your CEO is your direct boss, you should prioritize him first and allot time to other managers whom they want your help.

You should schedule your time to be able to satisfy all managers including you big boss.

If all efforts have been exhausted, then I think it's better to say outright to the managers that you can't attend to all of them.

Really it may depend on how difficult the tasks are. But at the end of the day, its not how fast you finish the task but how you did it correctly......Hope my suggestions make sense to you...