Driving a bit more with it today, my placebo brain tells me that the MCU1 performance is actually "fine" for day-to-day use on the current firmware. (I say "current" because I'm not sure anything is radically different in 2019.24, but I specifically poked around a bit today and didn't notice any speed issues.) Sure MCU1 won't play Beach Buggy and the web browser is slow/outdated, but if it manages to run at its current speed for all the usual in-car tasks (HVAC, map, music, settings) then I personally see no reason to rush to upgrade to MCU2, if that ever becomes an option.
(I also confess that I haven't been in an MCU2 S/X or TM3 recently, so maybe my brain has just learned to deal with the slight jitters and hiccups of navigating around MCU1. To be clear, MCU1 is not modern iPhone/Android smooth, but it's not painful either, like it had gotten shortly after the v9 update.)