I was a bit concerned about even the slowest speed being too fast for people, but at the same time I didn't want to suffer through an even slower speed when playtesting. Based on what I see in the comments, perhaps I should have lowered the speed further. I'll be doing that in the post-voting update, I guess.
...Actually, having a pause at the start of each level could help too -- a larger pause for the slower speeds and no pause for the fastest.
The slower speeds weren't made for more experienced players, anyway -- the idea was to let you pick a speed that felt fun for you. Thanks for the data point and suggestion!
Rust + Macroquad is, ironically, the game development tool that I'm the least rusty with at the moment. I've been programming primarily in Rust for quite a while now. It's a pleasant language to use, especially with its convenient sum types, expression-oriented nature, and if-let/let-else for control flow!