The top speed of the SR-71 was limited by the ability of certain components to handle the heat at MACH 3+ speeds. The engines were capable of more. It’s all about material science.
The top speed was faster than the official cross country record set by the SR-71 at the Air and Space museum annex on its delivery flight. They had a fuel pump problem during that flight slow them down. I was told that by someone who was there when it took off. He also said some in the program had thought about pushing it and setting as fast a record as possible. They called up the engine manufacturer to ask what the real engine temperature limits were, but finally decided that delivering an intact bird to the museum was more important than showing off its limits.