Just checked, fast slimes are certainly slower then the Player in the Open field, but they make up for it a bit if you try to run circles, since they don't need to circle around as far. So in my case it was probably just me being too bad at the game and making too small circles. However it is kind of difficult to make big enough circles to kite them indefinetly once enemy bullets and multiple spawner sites are involved I think.
The game is definitely meant to be difficult and challenging, and I am not certain I make that clear enough. My descriptions of the difficulty of the different levels might also be misleading (hopefully not too much). As I recall, when I uploaded to DD39, the difficulty was a rather tall difficulty cliff and not a difficulty ramp 😃 , so I have definitely messed up difficulty levels in the past, and I might well have to adjust it further. Maybe mix in some easier levels earlier that teach and reinforce concepts more?