If during a boom the economy picks up significant malinvestment (inefficient or oversubsidized firms, firms that were in a particularly good position to benefit from low-interest loans but weren't necessarily the most competitive firms that should have gotten it, etc. etc.) most of those firms are going to survive until there's a significant bust. China has artificially propped up a lot of industries and firms, and over time the deadweight loss of decisions like that builds on itself like barnacles on a ship, getting worse and worse. While I personally don't think the leadership is smart enough to want a slowdown, it might be a really good thing for their economy in the long term.

They dont, they are saying they want something they can't control happening . Old trick

I guess sudden economic growth can disrupt a lot of things, including potentially the structure of power.