The Adjacent Lane Speed Adjustment aint new.
The existing functionality you describe is Autopilot yielding for an upcoming merge introduced with the release of Navigate on Autopilot (but doesn't require Navigate on Autopilot to be active). I agree that the current yielding behavior is not great and I think similar to your experience, it can unnecessarily yield to vehicles in an adjacent lane that are already exiting. Around here, there's several exits that are not "exit only" but immediately after the exit, the right-most lane ends/merges, so Autopilot slows down until it realizes the vehicle it's tracking is no longer adjacent.
The Adjacent Lane Speed Adjustments is definitely new behavior with new display as Autopilot is not yielding (i.e., slowing down to match the adjacent vehicle to allow them to merge in front) but instead is making sure the speed differential between you and the adjacent vehicle is not so great while it passes these slower vehicles. From various videos, it seems like this behavior slows the car to be at most 10mph faster.
It's still unclear how proactively this will trigger especially with some places with HOV lanes separated with regular dashed lines, and it probably wouldn't be good if Autopilot slowed down significantly to match stop-n-go traffic. But then again, cruising at 70mph while the adjacent lane is going 20mph can easily result in an accident when someone slowly enters into the much faster lane.
