There are a lot of different variables when considering macro speed.
The first is your latency.
The second is the type of class you’re using the macro for. To be exact, you wouldn’t use the same speed for a Havoc Demon Hunter macro as you would for a Balance Druid macro. The reason the speeds differ is due to a higher probability of the cast being proced. The “filler” as you mentioned, are typically added to ensure a proced spell is cast if the rotation for that particular macro is fortunate enough to have a transparent and minimal rotation, such as a Havoc Demon Hunter. Therefore, the speed at which the macro would fire for a Havoc Demon Hunter, would be as fast as logically reasonable to ensure those procs are processed and not skipped by an additional spell.
Again, it all depends on the macro. Some macros do better with 200+ and some do better with -50.