Different kind of books tolerate different speeds. "Easy" books (e.g. fairy tales/children books) are more forgiving at varying speeds. For more "difficult" books (e.g. older books/philosophical treatises) you have to kind of work your way up and then back and so forth, they demand to be read at a certain speed. Here the famous quote of Pascal applies more strongly: When we read too fast or too slowly, we understand nothing.
"Poems" are a great way to methodically slow down your (forced) reading speed and be aware of it. Like it is much harder to play eighth notes spot on at 40 bpm than at 220 bpm.
Maybe it is beside the point but I would also recommend modulating your reading speed by choosing different kind of books, at least for me this is more effective and easier than trying to read say LOTR "slower".