My thought is AP is perhaps just one other variable in addition to all those previously discussed. Some of my technical assumptions could be wrong, but:
- By all appearances to some of us owners, Tesla does not appear to have set a hard limit on the number of USB tracks it will attempt to scan and use with Media Player; Rather it tends to keep going until it runs out of resources (memory) and is likely the reason some have seen it becoming slower-and-slower after processing some number of things -- sometimes, but not always, to complete hours later. To support this theory, we know for example, that too much Nav History impacted previous 7.1 stuttering and other USB-related items -- erasing significant numbers of old Places helped workaround those problems, IMHO showing the memory is shared between multiple subsystems including Media Player. I would think things like numbers of, and amount of data associated with Contacts, Calendar, etc are even more variables from owner-to-owner that likely consume portions of this fixed memory. As to which has priority over the other, I have no proof, but it does not appear Media Player USB is at the top of the list given the symptoms many owners have encountered.
- It's more noticeable with larger libraries, but USB scan/rescan of the same device generally takes longer and is more variable when one is driving (Nav is going, perhaps AP is active, you're playing FM or Slacker, etc., etc.) than when MS is sitting, parked and otherwise OFF -- IMHO showing that Media Player is using a processor shared with subsystems that become busier and have a higher priority as the vehicle is moving.
- Isn't the MCU/CID physically the same in more recent AP and non-AP MS? If so, IIRC from one of our resident bench testers, it has 2GB of memory, and the same processor and memory is responsible for a myriad of subsystems including Media Player.
Assuming all that is true, I then extrapolate if you have AP, it would be consuming more of the available MCU/CID memory for some of what it does -- tiles flowing, more logging, dynamic processes, etc -- providing less opportunity to fit the exact same amount of user supplied USB data a non-AP vehicle could. I suspect even with AP having some of it's own specialized and dedicated processors, some CID/MCU processor cycles are still consumed by AP functionality being enabled.
How big is the impact of AP on MCU/CID memory/processor? None of us could likely perform an exact enough test to say -- perhaps it's small, but I doubt anyone could say it's zero. Just to be clear, IMHO, AP isn't THE issue with Media Player. It's just one more possible contributing factor why owners without AP may be able to fit more tracks into an otherwise similar MS that does have AP.
...just a theory. As you say, it's food for thought.