I really like the concept. Super Hot with a sword has a lot of potential. I did have some issues playing it myself. I'm not sure if it was just my browser but I seemed to die if enemies finished their attack animation while I was within a short distance of them, even if I was fully behind them and avoided their spear. The best strategy I found was standing on the raised platform with a charged swing so the enemies came to me. It felt a bit unintuitive that the spear model hitting me didn't kill me but if they finished their animation it it even if they missed. But again, maybe that was just me.
I agree that some music would have been cool, especially if it slowed down or altered somehow when the game went into slow motion. Adding effects would make that transition more clear as it was a bit jarring early on trying to figure out what triggered it.