There's no difference between how the Properties work in general on different objects. The difference is how each object uses each of its properties.
Each time a processor receives an item, it gets its ProcessTime property to determine how long to process that item. If you adjust the ProcessTime during the processing of an item, it will have no effect on the current item because its events aren't re-evaluated by changing the property. The next time the processor gets the ProcessTime property (for the next item), it will return and use the new value.
Each time a conveyor resets, it gets its Speed property to determine its default speed at time 0. While running, you can modify the conveyor's targetSpeed, which will adjust its speed based on the current time, its current speed at that time, and its Acceleration/Deceleration properties. This will affect the events of the items on that conveyor. Its Speed property is only evaluated once, at the beginning of the model run. That's why its documentation says "The default speed of the conveyor." (emphasis on default.)
If you call setProperty() during a model run, then that property will be different whenever the object next calls getProperty() to get it. If the object doesn't evaluate that property anymore using getProperty(), then your change isn't going to do anything.
Phil BoBo
Sr. Manager, Software Development