I have never had any game slow down my Series X before. After waiting so long to play Eiyuden Chronicle: Hundred Heroes, I cannot believe how bad this slows down my entire console while it is running. Even the animation on my Dynamic Background begins stuttering until I close the game out...
I had two Sega CDs game that were in new looking condition (likely were only played a handful of times because, yuck) that were dead about ten years ago when I tested them before selling. So your mileage may vary. The rest of my games still worked so it was not the hardware.
I expected it to open up more than it did. I am like you and really enjoyed it for the first ten or so hours. By the time I was at the end I was just running through to finish it up so I could sell it and go play something else.
I thought the same at first. I think maybe it is just some other bug they are having. Sometimes I could get the points to register by just clicking the trending tiles. Sometimes it wouldnt. I thought I was on the 15 minute timer and was going to stop doing these. Then all of my points appeared...