There were confirmed reports that bringing up the drawing Easter egg (which suppresses the map background) would cause the stuttering and lag to stop. Also, after some distance and time, dismissing the drawing app caused the nav arrow to be incorrect, and it would slowly migrate on the map to the current location. This indicates that the Easter egg app is, indeed, keeping the nav from updating, suggesting that the nav is the cause of the stutter.
I confirm that in my case (70D w/ MCU1, v9 2018.48.12.1) I can still hear occasional stuttering/audio glitches in Slacker streaming audio if the map is in the background partially visible behind the music player and rear camera near the top. This only started for me with v9 - it seems improved slightly since initial v9 versions I had, but still not 100% fixed.
OTOH, I have yet to EVER notice the same audio glitches if I have the easter egg sketchpad obscurring the map (likewise in the background with music player and camera on top), even if I drive for days with the map hidden. I have tested the two cases a lot (map visible vs hidden with sketchpad)
Also, while I did originally have some severe touchscreen lagginess in some specific USB music player screens starting with v9 2018.40, that problem went away with 48.12.1 update. So I can't otherwise speak about any other touchscreen lag in current v9 version. There does however remain random font corruption and occasional screen lockup for me with 48.12.1
The Slacker audio glitches are brief and you might miss them if not listening carefully (or have a tin ear), but they're there, as I said, ever since v9.
My car was recently in for service for a couple other unrelated things. I asked Service to look into the Slacker audio glitches, but they "could not reproduce" the problem, even though I let them keep the car an extra day so the technician could take it on a drive overnight to/from work. They instead claimed it was perhaps due to LTE signal briefly dropping out, and suggested they need additional data - asked me to log the date/time of future occurences and issue a voice bug report. For the record I've noted the LTE signal icon at times of the audio glitches and the problem occurs whether 1 bar or at 3 or 4 bars so I doubt signal strength is the real cause - and anyhow it doesn't explain how the problem goes away when you obscure the useless map. (btw I've never noticed the nav arrow doesn't get updated while the map is hidden with sketchpad... I'll check that later.)
after picking up my car from service, almost immediately experienced the audio glitches again - and recorded several voice bug reports to mark when they occurred, followed up with email to service with time/date of the glitches. But as expected, just crickets - no followup response from Service - that's been my usual experience trying to report various UI or music player bugs over the past 3 years...
I listen to music 100% of the time while driving, so I wish they'd fix this and many other several-year-old
bugs in the USB music player - meanwhile I certainly don't need or want maps visible 100% of the time, nor do I need the amusing but otherwise mostly useless easter eggs (with exception of the sketchpad, which fixes the Slacker glitches!!)