Vitali didn't have great speed but could definitely crack. Ingo is another guy that was not a burner. One guy that comes to mind that is a little on the unknown side was Maselino Masoe. He held an alphabet title at middleweight back in the mid 2000's. he was devastating puncher but had no speed. I saw him fight live a couple of times and could feel his power from my seat.
Most heavyweights that hit hard were pretty slow. Most people are under the delusion that Foreman and Liston were fast, or at least fairly fast, but tbh, they were two of the slowest boxers I've ever seen. Mariano was also pretty slow.
Danny Lopez was pretty damn slow overall. He definitely qualifies. Not quite as slow as the other two I mentioned, but an even bigger puncher.