Valgrind is fine for teaching students. I would probably use something faster and more sophisticated (and probably more expensive) in a professional setting but that doesn't mean there's nothing to learn from using it.
Adding logging also changes timing. Those aren't just NOPs you're throwing in. Also, changing timing isn't always a bad thing. Sometimes you can get a bug from a race condition to happen more often with the debugger.
Valgrind can't be fast, it has to track/color each memory location, it also has to check each pointer reference; also its memory overhead for larger footprints can be considerable.
Once upon a time it was really slow, but then they added just in time compilation.
Adding logging also changes timing. Those aren't just NOPs you're throwing in. Also, changing timing isn't always a bad thing. Sometimes you can get a bug from a race condition to happen more often with the debugger.