The few STP files I’ve imported into SketchUp have had the same issue. And it’s like STL files in that regard. They are heavily triangulated which does tend to slow SketchUp down. You can spend some time deleting unneeded edges but you’ll probably need to do it manually. It’s likely that any cleanup type extensions will have difficulty deciding what has to be kept and what can be discarded.
In most of the cases where I was given an STP file, I found it easier to draw the object from scratch in SketchUp than to clean up the file after importing to make it usable.