Not sure if this is still an issue. MCU1 used to have problems if the navigation history of past destinations got too long - which caused performance problems for all of the software running on the MCU1. Manually clearing the navigation history fixed the performance problems.
If that issue is still present, that could contribute to the perceived faster performance of the MCU2, because the settings/history are reset when upgrading from MCU1 to MCU2.
At least how we use our 2017 S MCU1/HW3 and 2018 X MCU2/HW3, we haven't noticed any significant performance differences, and because the Infotainment Upgrade primarily provides new functionality only while the vehicle is parked, we won't be upgrading it to MCU2 and then lose our FM/HD/XM radio - which is used during much of our driving.