Cute and fun! There does seem to be a glitch that can happen where if you try to save time by switching to another character while another is slowed down, that character is stuck slow forever as though the black slime inside of them is still there despite the UI being empty.
Self-indulgently, I do wish you could "overstuff" the characters a little to carry more slimes at once but be slower for each one over your "carrying capacity".