My regular script is ok, but once I write it quickly, it will not look good. So what should I write?

I can't wait for this kind of thing. I have to practice slowly. I practiced for nearly ten years before I could write fast and well. Running block letters also evolved from block letters. Don't write too fast yet. Take a word from write slowly, choose a nice font, and keep writing until the turn is natural. It's basically ok. I can only express it this way, hoping to help you.