havent had a chance to read this ENTIRE thread yet. But as someone with a 2017 90D with 5500 miles on the car, and who recently went to HW3 FSD upgrade but kept his MCU1 (for now), seems I may be noticing a pattern.
When I have my USB (Sandisk 128gb thats on Tesla's recommended list) plugged in for dashcam/sentry? Appears my MCU is slower. Did some informal testing and finding that when I have the USB plugged in, voice commands (for example) are very sluggush as is refresh of the map.
When I remove the USB, after a few mins, the voice/map refresh becomes much quicker. Almost pre FSD upgrade levels.
This all in my mind, or has anyone else observed this?