My thesis is basically holding true. Haven't noticed any timer slowdown since I started periodically closing and restarting the game (plus not leaving the game open while in Rest Mode).
Sometimes I do feel like I'm getting input lag, even without the timer situation changing. It's odd and it's hard to figure out because I'm definitely not an elite Tetris player, so when my results change my first thought is that it must be something mental on my part. But watching Blink's Twitch stream the other day, I noticed he said something about feeling some lag and needing to reset the system. I didn't get a chance to ask for more details (plus he was streaming) but I really know nothing about how input lag on Tetris Effect or Tetris titles in general works.
I'm also hooking up speakers to my monitor and it *feels* like I don't have that aforementioned input lag when I do that? Again, very hard for me to tell, because my results are still all over the place, but it distinctly feels different. More responsive.
I found a video of my SS run in Master and confirmed, to my despair, that the timer slowed down in that mode too. Even though it's not a time-limited mode, I feel like that means there was significant game-altering amounts of slowdown, and I didn't really "earn" the SS. Back to the pile it goes.
On the other hand I easily redid SS rank on all the other Effects modes (except Mystery, which I haven't bothered with. ugh).
Got my "legit" sprint down to a 1:13. Hopeful I can get the SS for real. I've never even approached this level of play, so I fear if I take a week or two off it'll all leave me
